The IAP model became dominant on mobile largely because of Apple's store policies and their refusal to allow other forms of distribution. Meanwhile Steam and many other channels are largely pay-once, and they exist only because of the developer freedom that you decry.
No, the IAP model became dominant because the unwashed masses can’t be bothered to spend a few dollars or even just 99¢ for a copy of software that oftentimes took many months if not years to develop, to say nothing of on-going support and maintenance costs. Too many people (in general) are just flat out fucking cheap and too many devs/companies are willing to resort to stuffing every app with ads and tracking.
