Because capitalism doesn't work without taxation, not in the long term. This probably sounds weird to most people, but it is the truth.
In capitalism money naturally accumulates at the top, but the whole system depends on money changing hands regularly. It's the essence of the economy. With no taxation all money ends up at the top with people who have nothing to buy, the system grinds to a halt and you have total economic collapse. In order to keep the system working the rich needs to keep feeding money back into the bottom so it can trickle back up to them. Human nature being what it is, you can't expect people to do this on their own. A few (Bill Gates for example) will, but in the majority of cases they would rather sit on the money so it can make even more money for them. Giving to the poor is not "fiscally responsible". Plus it would be very inefficient to make every multi-millionaire/billionaire/multi-billionaire manage their own welfare system.
That's why it is the government's job to redistribute wealth downward to keep the economy healthy. We have endless examples of governments that failed to do this and ended up with stagnant economies and endless economic depression. Look at feudal European economies for example, where the lords hoarded the cash and left charity to the church.
If you want to see this in person, crack open a game of Monopoly. The winning state in Monopoly is total economic collapse where one person has all of the money and nothing to spend it on. If you want to make it more realistic start the game with one player having $20,000, two with $200, and everybody else with $1. That's how real life economies are. If those $1 players lose tell them they didn't work hard enough and that they should pull themselves up by the bootstraps.