While you are correct that "income taxes do not stop people from trying to better themselves financially", an important nuance is that it also massively incentivizes how they try to better themselves financially. This is evident globally and often has unintended consequences.

Generally speaking, you want to incentivize people to try to better themselves financially by pursuing highly productive and legal activity. If the best way to better yourself financially is rent-seeking, becoming a government bureaucrat, corruption, or participating in the gray/black market, that is what you will get.