Apple did this to itself. Reportedly it was Jobs' opinion turned policy that Apple don't do games or pornography.

Exactly this policy and their interference to app developers created a selection pressure and a cutout hole in shape of "only slightly gamelike && technically not pornographic && in high demand", and the category of apps more accurately represented as "strip clubs with casinos with no cash-out" filled the vacuum like a Ghibli film blob monster.

Early iOS games were more game-like. Apps like SNES remakes, flappy birds and music games, were more common, but they all converged down and down into porn territory.

It doesn't happen naturally; not even pornographic game markets, let alone Steam or Itch, aren't as badly infested with gambling as App Store. It only happened artificially by how Apple ran it over the past ~15 years.