> Instead of that, why not make it so that an app can access SMS / RCS only when that option is allowed when you have a special Google Account?

Because then you still need a special Google Account to install your app when it needs to access SMS / RCS.

How about solving this problem in a way that doesn't involve Google rather than the owner of the device making decisions about what they can do with it? Like don't let the app request certain permissions by default, instead require the user to manually go into settings to turn them on, but if they do then it's still possible. Meanwhile apps that are installed from an app store can request that permission when the store allows it, so then users have an easy way to install apps like that, but in that case the app has been approved by Google or F-Droid etc. And the "be an app store" permission works the same way, so you have to do it once when you install F-Droid but then it can set those permissions the same as Google Play.

It's not Google's job to say no for you. It's only their job to make sure you know what you're saying yes to when you make the decision yourself.