They could preinstall a bunch of competitors and you'd choose the ones you keep at first startup (like Microsoft had to do with browser). This would allow fairer competition for competing stores.
Also they need to make sure the playstore is not required for the phone to work correctly which I'm not sure is the case on stock Androids currently.
You can disable them, which is functionally uninstalling. Actually being able to uninstall them isn't technically possible because their base images live on a read-only partition so they survive factory reset (which erases all RW partitions). The OS could _pretend_ they're uninstalled, but that seems strictly worse than the existing presentation which more accurately reflects reality.
Not exactly the same situation, but I keep wondering : is deleting personal data only "pretending", since most of it could potentially be restored for quite some time (especially on transistor storage) ?
Also they need to make sure the playstore is not required for the phone to work correctly which I'm not sure is the case on stock Androids currently.