> The company that’s called Unity is in the business of selling ads and showing lots of them to people on phones. They own a game engine as a delivery vehicle for those ads.

but if that were really the case, why would they start charging per install? Why wouldn't they distribute the engine for free, but with the caveat that free users must show the ads?

Adding a revenue share per install is the antithesis of trying to get an ad platform more widely distributed imho.