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Being open source is a red herring in this instance.
Say that Chrome implements a web feature you don't like. You fork the browser and remove that feature. But websites expect Chrome, and they use that feature, so your fork doesn't work with those websites.
Say that Chrome refuses to add a feature you want. You fork the browser and add the feature. But websites expect Chrome, so they don't use your feature so as to not break for their Chrome users, and your fork is no better off.
The insidious part of a web monoculture is allowing Google to dictate the standards of the web platform. Being able to fork the codebase only gives one the power to change things that are strictly client-side.