The human body is subject to a number of reductionist approximations which allow us to work with it predicatively. These approximations and models are extremely well supported experimentally. So far, the only such approximations that apply to the economy are the Keynesian ideas, and those produce results that are intuitively nonsense, and have no experimental backing. Modifying the economy without causing more harm than good is much, much harder than it is in medicine right now. We are not surgeons here. We are plague doctors with masks, and leeches.

And yet we've gotten past leeches, and developed medicines that work. Don't you think we should try some economic models to see if they work? Who cares if Keynesian ideas don't make intuitive sense? You just explained why that the economy is too complicated to understand intuitively. The evidence isn't very good (for or against), because it's never been properly tested.

property rights are fairly straightforward.

No they aren't. How do people come to own property in the first place? (are the descendants of native Americans due reparations? Who has the land rights to Israel / Palestine?) How much do polluters have to pay for the damage they're doing to everyone else's property? Do I have the right to sell myself into slavery? What ideas can be patented (copyrighted) and when do those patents (copyrights) expire? Unfortunately, to try to sort out who owns what, is to mire yourself in as twisted and complicated a political issue as any.

Additionally, the government is fantastically bad at spending money. Orders of magnitude worst than essentially any private organization. I've seen the way they buy chairs. It's terrifying. In general, putting them in charge of any significant sum of money is an excellent way to ensure that much of it will wind up being expended as economic waste heat.

Be specific. How does the government buy chairs?

Also, overpaying for something doesn't make the extra money simply disappear as "economic waste heat." The people who sold those chairs can spend the extra money again. The actual cost is bad information being fed to the market, which can result in workers making too much of one thing, when everyone would have been better off if they'd made something else. And it's not as if private consumers always spend their money perfectly in accordance with their values either. If anything qualifies as "economic waste heat", it's the tobacco industry.