They already kinda are. Look at, say, Drive Now (https://uk.drive-now.com/) - they're owned by Sixt, and they operate Drive Now jointly with BMW (BMW cars being the only type you can have with DN). I imagine automakers have relationships with other "car-sharing" firms (e.g. Zip Car) as well.

You might already know that, but for everyone who doesn't live in a city where they're available... :)