The web doesn't run on its own platform. It does run on browsers, which are controlled by few careful instances. Mobile browsers are subject to the same rules as any other app and most popular browsers are owned by lower level platforms, separated from website/webapp owners. It is not the same as side-loading. Web analogy would work if websites were executable binaries or if browsers were much less restrictive. It is an open ecosystem under a strictest environment ever made. Nobody is going to download your random binary as mindlessly as they tap on a-hrefs.

Following the idea you present, Epic should just start making browser games with in-game purchases just over a credit card. Why doesn't it then? What's wrong with the web that Epic couldn't just publish on playfortnite.com and that'd be it?