What person, who is good at something, does that something for free?
That drunk chick at the shady bar (the kind you find all over the place and really don't know anything about) will fuck you for free (minis the cost of drinks, and cab fare, and her cab fare back to her house). However, the fucking is sub par, and when she starts puking it gets annoying, you now need to purchase a new set of sheets. If you made the mistake of giving her your real number and taking her back to your real place then she will harass you until you give in to at least 1 pity date, which will cost you money. And you better hope she didn't get in with a fake ID, because getting but-raped for 10 years isn't worth 1 free lame pukey fuck. Yes the free fucking ended up costing you, like open source eventually does.
At least whores are efficient, they work correctly and better than random sluts you find all over the place, and you know the cost upfront.