While the jokes on here are pretty funny, I think a serious answer is overdue.
The answer, as many have jokingly alluded to, is the idea of “getting paid to do work.” Would you be willing to work at McDonalds for free, especially if you have expenses to pay every month? Probably not.
Of course, there’s a crucial aspect that hasn’t been talked about yet: charging for games means locking out people who are unwilling or unable to pay for them. It is kind of sad, especially in the latter case, but that’s how society works.
And, uh… I honestly don’t know what the money tree part means? I feel like there’s a bit of a communication issue here.