Why would they do it?

Because they're experts, they know all the answers, they can fix everything and don't need any money to do it.   Let's seem them do it.
 
I'm up to ears in debt, because my last dairying boss that I was sharemilking for missed his target by 20% production, leaving me out of pocket by 10k, and that's a lot of debt to be going back on a sickness benefit with.   So I knuckled down and took on the challenge, I put my earnings from farming (negative eight thousand dollars) and my accumulated equity together and brought a herd to give me cashflow.
  I had hoped to make some improvements from what I know from outside the industry to get ahead.
But. My old boss was going to leave 80 cows with me until people started drying off (and thus selling surplus winter milk cows)...but he got a good offer from the works, and sent them off 3 weeks before my sharemilk contract finished (let alone my milking one).  Then the guy who promised me use of his tractor for the first winter decided that, well, it might break and he didn't want to risk that, and so I had to find a tractor straight up day one, on credit. One reliable enough to get through the winter.  But I thought...no probs, IRD owe me about 30k in GST return for the startup costs - but they didn't payout.  IRD hold on to large amounts for checking to make sure it's not a scam (which a lot of carbon credits guys were pulling at that time - invoicing the purchase of a farm, collecting the GST on the invoice, failing the purchase, closing the company because of the purchase failure. paying themselves wages for time worked out of the GST cash. folding the illiquid company).  And I've asked around - it's not uncommon type of problem.
 But that's the thing.  It *IS* possible to startup a dairy farm business from 0 or less cash.
So those armchair "experts" can do it.
 
And they can do it, and use their financial expertise to calculate their way through it.
 
And they can show me how to achieve all these things they want.
 
Because it's all fine for those who have had years to establish their business (and as you say, you've been exploiting/profiting the environment the whole time) or who have large gifts from outside (eg Landcorp).
 
And this land is not right for growing vegetables.
I'm motivated by (a) making a reasonable profit as a normal business operation, and seeing that is sustainable throughout the industry (as opposed to "steel disease").  and (b) setting up sustainable, intensified systems, that can be staffed and operated in a modern way - without having to give every government crony and consultant their "graft" to do it.
 
After all.... I've been at this 4 yrs now (with a couple sharemilking beforehand).  And the farm has been seriously broke the whole time........
 So What's stopped you folks doing this environmental improvements off your own back before now???????????????????????????????????????????????????????????  (and you say the environment was better off with you..... I was _already_ fencing off streams (for inline dosing), I have had zero inductions policy since I started sharemilking, and I already use under 2 ton N for 60 hectare!!!!!  I already use heat recovery on my hotwater and this shed has had solar hotwater for 20 years.   let me know when you "environmental friends" catch up.!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!)

 
And my challenge still stands.   They think they can do better, buy the cows and put their ass on the dirt and do those hours in the shed (how many hours you spend cupping and washing up and driving tractors this year Kate?) and then they can make the calls.
I've paid my dues.  Let's seem them sing the same songs when they've paid theirs.

SimonP, "User pays" means that CONSUMERS receive the passed on costs.
And hey, farmers are all for that.

But that's not what you're saying, you are ignoring that the farmers (the guys you're saying "don't make sense") are price takers.  The Retailers, Distributors, Wholesalers, and Processors (and more importantly their Information Economy parasites, make all the money, leaving pittance passed to farmers.   And you're indicating that farmers be the ones responsible - but it's not the farmers that set the sale price of their goods, at least not in Dairy and Meat, and cropping/vegetables are lying low because they've got even bigger problems than ruminant farmers.  The Processors are the ones who decide how much they're selling for then take their expenses off, and pay the farmers a portion of whats left.
 
If you're looking for environment improvement, you're either looking at a socialist system (for the countries good, and everyones' expense), or User pays which means the shelf prices must go up considerably for all meat, milk, grain, vege, fruit, and textile (eg wools) (or we could import them o_O?!? from people without environment constriants!), or for a few people like myself bucking the wholesystem to find out what works.  Although there will be considerable numbers of NZ farmers who will just buckle down, invest more capital in business and say "they're not coming for me, so it's not my business".......

"The only possible argument for is that farming creates significant economic benefit to the country and the individual farmers"
This is why I labelled it socialist SimonP.  Claiming it's all ok, just because a bunch of do little townfolk get to ride in on the gravy train, is not acceptable.
  The only possible argument is - the do little consumers part with resources to either fix the problem and see that those resources actually get used to fix the problem (not paid to consultants, celebrities, councillors, or other parasites.) or there is no other option (or if serious enough, the activity is halted - eg Exxon, fracking)
 
Saying "we're all getting rich from the pollution".   well those that do that deserve to wallow in their own outcome - shame they have to take the rest of us with them.
 
But the population is going to go with a price competitive market, and a free market favours lowest cost producer with the best Information Liar.  Which results in best premiums to clean up ending up in pockets which don't spend, especially on clean up (and usually on better liars or protecting the ones they already use).  
So it comes down to User Pays, but that has to be the consumer end.  Force a business to become a consumer, then that business loses value, fails to pass on shrinking revenue to staff, and eventually folds - because the business doesn't really get a choice in the consumption.  If a sale needs to be made, then they need to produce.  It is the consumer which makes the "point of consumption" choice.  Either they can afford to consumer - or not.  If a business is forced into a consumer role, it would be forced to close that product line if they can't afford to consume.
 

Belle, they might be finding what I have.   Such things work ok for lifestylers ,who just want to do that kind of thing for there own amusement and/or survivalist philosophies.  But in this day and age trying to actually turn a living of such a small acre is unlikely - and moving to exotics (olives, nashi, etc) or niche products is quite expensive and high risk in the current economic climate (read: very difficult to establish market connections.)
 
I suppose could do wheat and sorgum seedlings, or even kick off a industry starter providing starter kits for (sub)urbanites wanting to get their own vege/herbs going but those are rather full markets.
 
As you say 7days a week, and small size means few staff options.