Why are mobile game ads for other mobile games?

I play a lot of shitty mobile games and get non-stop ads. The overwhelming majority of these games advertise other mobile games, and just a small number of ads are for stuff that actually involve me buying anything.

The mobile games advertised also rarely involve any kind of payment, 99% of them are free to play.

I just don't understand the money making incentive for these ads. It seems to me like they just loop around.