The free games are 80% shovelware not worth playing, 15% indie experiments that have the potential to become a full game with another development iteration, and 5% AAA games that can be bought on sale for a fiver anyway.
I doubt much of their Fortnite money is actually being spent on licenses for these games. They likely negotiate some kind of “do it for the exposure” deal with the smaller developers in order to keep the flow of free games going.
Chances are the games given out for free will end up in a Humble Bundle at some point anyway. Which is when you acquire a steam key anyway.