If they supported things like PWAs and other mobile web initiatives, people could build applications that run seamlessly on their phone, look like native apps etc, and they wouldn't go through their highly lucrative app store.
As phones get more and more powerful and things come along and mature like webgl etc, the reliance on native apps drops more and more. As does Apple's control.
Maybe I should get back under my tinfoil hat, idk, but that is my guess as to where their reluctance comes from.