I've found that as people, especially men, age, they can get very set in their ways and stubborn. My father insisted on attempting to undertake things in his late 80s that he shouldn't have been doing anymore ... just because that was what he did. He developed behavior patterns that it was impossible to shake him out of.
Now, your primary responsibility is to take care of your own family financially, and you have an obligation to take care of your parents ... in terms of the essentials. So, if it gets to the point that your parents can't afford food, medicine, clothing, or other essentials, then you need to try stepping in. If he won't be persuaded from selling off pieces of the farm to maintain his lifestyle, then there's nothing more you can do. I feel that you would be doing wrong to your family to give your parents money so that they can "buy presents" and the like. Buying food and other essentials would be a different story. So I personally feel that you should NOT give them anything. You'd be taking away from what you are obligated to give to your own immediate family in order to give your parents something that you are not obligated to give ... extra spending money.
That's the thing, they're already "set" and never have to worry about this again because what they already get is enough for all their necessities, but my dad insists on working the farm and they don't budget or keep track of anything.
Yes, he's terribly stubborn. He says that if he didn't do anything with the farm, he "wouldn't have anything to do" and that the farm would go to waste, get overgrown etc. THAT'S his main problem, this idea that the farm is all he can or should be doing, even if it means being broke all the time. It's not true anyway, because he barely spends any time in the farm or with farm-related things. He spends his day mostly watching TV and flat earth videos. These other things get my mom even angrier.
I'm sure it's also about pride. He doesn't want to face the facts and admit that he cannot manage it and doesn't have the capital. Agriculture is all about capital because things take years to give a return, and even if everything goes 100% according to plan, the harvest can still turn out bad, yield less, a plague can set in, the price changed, etc. A million things can go wrong in agriculture.
He would rather be broke than rent the land and have some else capable make it be fruitful.