If you have something, an item, a service, anything, and people think it has some value or worth to it, or they think they need it for some reason, of course people are going to make money off it instead of getting nothing.
Knowing you helped someone isn't worth an empty wallet to most everyone.
Why do we pay for it? Once upon a time someone probably decided to sell what they knew to other people. I have no idea when or where, but it seems to have been made tradition.
That's my best guess, certainly not fact. Don't feel like looking this up at all.