Not malware, just ad revenue generators. Although I guess this depends on how widely you define "malware" since many of them are probably scraping, at a minimum, usage statistics while you play and possibly also device data and who knows what else.

You get 1-2 minutes of gameplay in between each ad and all of the "levels" are probably generated once by a program (rather than a human actually designing the level layout/challenges) to minimize startup costs. I'd be willing to bet that if you traced the ownership structures for the types of games the OP is talking about you'd find a handful of megacorps owning hundreds of them and just reusing assets and programming as well.

Then of course there's the sinister preying on your psychology in subtle ways to keep you invested just enough to sit through the ad the play the next level.