Brave and Edge are better than Chrome if you care about privacy, but 99% of the code base is still controlled by Google and they don't really have a voice in standards committees. To support the open web it's much better to have engine diversity too.
Or the conspiracy take: Mozilla only survives because Google wants to make sure that an alternative exists so they cannot be considered a monopoly.
That would also explain Mozilla behavior for the last few years... But motivations from big corporations are rarely as singular and neatly defined as I just made it sound, so while that might factor into their actions, it probably cannot be considered to be THE reason for action.