This is pretty much the same argument as someone saying "Why is the government allowed to put people in prison, but when I lock someone up in my basement I'm apparently the bad guy?!"
It has also always been the purview of government to tax the population/businesses whatever it can get away with, just as it has been the purview of government to regulate commerce for the benefit of its citizens:
https://en.wikipedia.org/wiki/History_of_competition_law#Rom...
You're free to disagree on the ethics, but hopefully you can accept the logic of it. If I may ironically steal a line from William Francis Sutton Jr., they tax the rich because "that's where the money is".
To be a little more nuanced, though, money has diminishing marginal utility, in the sense that $10 means less to a billionaire than to someone who is broke, and therefore if a tax system takes into account the perspectives of the people who hold the money, rather than the perspectives of the pieces of money themselves, it makes sense to tax not just as a fixed proportion of people's wealth, but to tax wealth above certain thresholds at higher rates.
Consider also that the median US household pays about $10k in taxes per year, and has a net worth of about $120k. That means that, just to tax billionaires proportionally (not progressively), there could be a wealth tax of 8% per year. Instead we tax income, which punishes people for working and rewards the idle rich.