What issue does it solve to give Elon Musk $500?
If a benefit is only intended for select people, there is a cost associated with administration and enforcement. That cost can be quite significant, but more importantly, opens it up to questions of who deserves it, and how much, thereby negating its universality. If rich people aren't eligible, who gets to decide where to draw the line between who is rich and who is not?
To a rich person, UBI amounts to rounding error, they literally won't notice the difference between getting it vs not. The idea is they'll be paying way more in taxes than they get from UBI.
UBI is superior to something like a guaranteed minimum income. With a GMI, you make up to the GMI amount whether you work or not - so what is the incentive to work? You end up no better off than if you don't. This is a big problem with any state benefits, though usually it's worse, like with welfare, you tend to lose benefits once you reach some threshold, so working too much or too hard leaves you worse off than before.
With UBI, it's a gradual trade- you can work not at all, and the state hands you let's say 40k per year. You get bored, or maybe you just want more/nicer stuff, so you take a part time job and work 20 hours a week - you still get 40k per year UBI, but you also earn 20k per year, of which 10k goes to taxes- so your take home is 50k, your cost to the state is reduced to 30k. As you work more hours and/or acquire more skills, your pay (from your job) eventually reaches 80k per year - at which point 40k goes to taxes, so now you're at break even as far as UBI goes, your take-home is 80k. You're still a net drain on society, (because of course the government does things besides pay UBI) but not as much. As you earn more, you pay more into UBI than you get out of it- but your take-home still rises, so people who want to pursue wealth as an end can still keep some of it.
Of course the numbers are pulled out of my ass, I'm not even attempting to come up with a working plan, just getting the gist across. I'll let people smarter than me hammer out the details.