Apple literally puts technical measures in place to ensure that it's 100% impossible to download outside software.
Google literally spent most of a decade preaching that the "freedom" their platform afforded to sideload software was a major competitive advantage, and then attacked a developer who was making use of that freedom to make a large profit without giving Google a cut.
Also, with Fortnite specifically, they proved the worries right immediately: https://www.forbes.com/sites/ryanwhitwam/2018/08/25/epic-gam...
Unfortunately, the $99 annual fee per developer account nullifies any possible mass-market sideloading possibilities. (Imagine what could happen if sideloading apps from source was free just highly impractical, and HEY turned their rage into a free Apple developer account management & sideloading SaaS offering.)