Being open source doesn't change the fact that it's the same codebase.
Why should there only be one browser engine? I'm a web developer and I hate cross-browser testing/compatibility, I prefer to use Chrome for it's devtools. I would hate for there to only be one browser in the world.
Duplication is not equivalent to standardization.
On the mobile space users are at mercy whatever WebKit version gets to be integrated into a specific OS release.