Why should that matter? If anything, shouldn't that mean that Google doesn't have a monopoly on Android apps in the way that Apple has on iOS apps because the device manufacturer can pre-install their own store on their devices? Like Galaxy Store on Samsung devices

It matters because Google got to the current state (Play Store installed on more devices than it would have been) through anti-competitive behavior.

Ergo, the redress isn't to say "Keep the ill gotten gains, but don't do it any more" -- it's to attempt to return things to the competitive playing field that might have existed if Google hadn't broken the law.

From that perspective, forcing them to use their market share to distribute alternatives makes sense.