One of the problems with homelessness is that it's a systemic issue.
Another problem with homelessness is that it's dozens of problems, because different people have different reasons for being homeless.
A mother living with her two kids in a tent could see most of her problems solved with a monthly cash injection; she could get an apartment, make sure her kids have sufficient care while she works, or just take care of them herself using the stipend.
The guy who sits on a bench howling at the moon because he's convinced it's implanted digital worms in his teeth? If you cut him a check, he's not going to be equipped to spend it in a way that's beneficial for himself.
You also don’t have to divide it equally among homeless people. You could start with a few and add more to the program over time, as the endowment grows. Those getting the money would be assessed based on potential to get off the streets permanently and find jobs, housing, etc.
Because then it pays for itself. Look at the Nobel prizes. Look at Harvard University. Look at the Norwegian pension fund. These are backed by HUGE endowments which pay for themselves with profits earned on the market. They are also extremely stable.
On the other hand, look at Social Security in the US. Any sort of benefits program paid for by taxes is exposed to political interference. That makes them inherently unstable.
But no, they can’t do that. It would piss too many people off. The problem with homelessness is that it’s always someone else’s problem. Everyone agrees that it should be dealt with, just that no one wants to deal with it themselves.