This is a silly argument. You cannot force others to use one product. WebKit is great, but it is still a project, governed by some people, and with certain goals. People want to create browsers who may have other goals, so either they have to convince the WebKit team to follow them, or fork, which ends up causing the same problem of multiple platforms.

We have open protocols and specifications. The web uses HTML, CSS, etc. Then it's up to the various rendering engines to adhere to those.