Given enough time, it does. People used to talk about a “WebKit monoculture” on mobile back when both Safari and Chrome were built on WebKit (although, they enabled different features and used different JavaScript engines). After Google forked WebKit to create Blink, WebKit and Blink started to diverge with different development priorities resulting in different improvements being made and different features being implemented. So as time goes on, and code gets added, removed, and rewritten, much of the commonality eventually disappears over time.