The problem, as always, is implementation. Millionaires, especially those on the low end of that spectrum, tend to be fairly easy to tax. Their income sources are mostly regular and standard forms of income tax work pretty well.
Billionaires are a very different animal. They have armies of lawyers and financial managers that help them navigate the movement of wealth through the global system to avoid taxation. They avoid recognizing income at all opportunities and strategically use debt to provide for themselves without exposing themselves to tax consequences that would come from getting the money they want to spend. They can traverse the globe looking for the most favorable taxation rates to maximize their wealth.
There’s been a lot of “tax billionaires” and “billionaires shouldn’t exist” rhetoric without much substance to how to actually achieve that goal. Billionaires aren’t just going to say “aww, shucks, guess we’ve got to pay up” when governments decide to up their taxes. They’re going to react in ways that are both hard to predict and harder to stop. Capital flight is a real threat and will require a massive, coordinated effort on the parts of all the nations that comprise the global financial system to curtail. The efforts towards a minimum corporate tax rate are a good start, but that’s only one step of many that are necessary before “tax billionaires” becomes a realistic target.