As someone else noted, this is due to the issue solved with GPL3, which redefined the roll of derived work and "linking": as it stands, WebKit *is replaceable, but doing so requires you to first defeat the cryptographic locks that prevent you from modifying any of the software at all.
Cue the apologists saying that Apple is saving the world by not allowing alternate HTML rendering engines on iOS. If only Apple would be so benevolent as to disallow us from installing other rendering engines on MacOS too. A man can dream.