If you promise consumers that your new platform is an "open" one, you are creating a new market where devices will be made by multiple vendors and antitrust law will apply.

Google chose accelerated platform growth in exchange for being bound by antitrust restrictions.

If you create a new platform that that customers know in advance is a walled garden, like XBox, you do not face the same restrictions.

That's how the existing law works.

If you don't like how the existing law works, you have to do what the EU did and change it.