Because it's not fair. The effort to feed and house yourself in reality only takes a few hours a week. The rest is rent (in the economic sense--no value provided; just paying someone who got there first and put up a gate).
But I believe that one purpose of civilization is to make life fair.
Not to give everyone an equal slice, but to give them an adequate slice so that they have a fair shot at having a healthy, productive, satisfying life. As someone who has more than I need (not by a ton, but all my bills are paid, and I have some surplus), I want to share it effectively at least with those who are struggling through no fault of their own.