A couple of major factors:

Users who expect low prices - This partly because of the history of mobile games being smaller and/or ad-funded but also because the vast majority of people playing games on their phone are looking for a low barrier to entry, time waster, not specifically a game.

Lack of regulation or enforcement - other gambling heavy fields tend to be at least somewhat regulated, but mobile games are very light on regulation, and even lighter on enforcement. This allows them to falsely advertise their games and how they function (both in terms of misleading ads, and lying about chance based events and purchases in-game).

Monopolistic middlemen - On other platforms, theres more direct competition (IE, Sony and Microsoft's generally more direct competition) or companies that prioritize long-term growth and stability (IE Steam or Itch.io). Apple and Google, on the other hand, largely compete on brand perception and hardware specs. These means that their app stores, where they make most of their money, have zero competitors. Seeing as they have no reason to make the stores better, they can instead promote whatever makes them the most money; that being exactly these manipulate, sketchy, virtual slot machines.