Why are inappropriate advertisements showing on free to play kids' games ?

I let my son playing free to play 4+ games and it surprises me to see those advertisements run on those free to play games are sexual contents or very inappropriate for kids.

For example: teacher simulator which shows up an AD after we finish a level. It shows implicit sexual scene where there is a shadow a dude and naked girl in a shower. Then another one pops up, a game shows a lot of blood and cut body parts. Even if it is comical but I don't think it is good to show those ADs.

I suggest any games that put ADs like that, should have 12-15+ years old restriction, not 4+