Primarily, I think, because an individual contribution/overpayment has almost zero effect in the big picture. It's the systemic issue that needs to be addressed.

This is like giving someone a bottle of water, when what they need is a well built.

A compounding factor is that billionaires are by nature, competitive. Giving away half of your net income puts you at a disadvantage against other billionaires who can use that against you. Taking away half of everybody's net income keeps the playing field level.