The wealthy get a million dollars along with everyone else, and that money has to come from somewhere, so you need to tax the wealthy a lot more than a million dollars on average. Basically the way to think of it is: you're giving everyone a million dollars on average, so you need to tax everyone a million dollars on average, and the only way to make that work is to actually tax someone with, say, $5M/year in income $4M of that income to balance out the people who make only $20K/year and can't pay anywhere near their share. (That tax rate is probably impractical, but then we're looking at a pathological case for the sake of analysis anyway.)

If it's revenue neutral, then that means you're taking money from other programs - which by definition spend that money. Just on different things. (And probably a lot of the same things, if you're taking that money from existing welfare programs.)

The more serious proposals I’ve heard are for revenue neutral ubi. Which means taking the money from current entitlements and giving it evenly to everyone. I’d love to hear a real economist who supports the idea and is in favor of paying for ubi by raising taxes on the wealthy. I do not know of any economists that fit that description.

On the contrary. Most of the "revenue neutral" claims I've heard are Americans making a politically-motivated claim to reduce libertarian/conservative opposition. Actual concrete proposals always require taxation - see e.g. the Swiss proposal (voted down in a referendum because of the taxation requirements) and this [1] overview in the Economist of the prospects of adoption in other countries.

Generally, if I see a claim that a proposal for UBI is revenue neutral, I assume they're either a) left-wing Americans trying to sneak welfare hikes by right-wing Americans, or b) right-wing Americans trying to sneak a drop in net income transfer to the poor past left-wing Americans.

[1] https://www.economist.com/blogs/economist-explains/2016/06/e...