Nobody really knows what company will develop truly practical SDC tech first. Therefore nobody really knows what revenue model that company will have.
If a vehicle manufacturer does it first, then probably their main goal is still to sell equipment, so they would probably be open to selling/leasing it to anyone and everyone.
If a tech company gets there first (or any company without an existing revenue stream), there's always a chance this company will want it all for themselves.
Sure, Uber and Lyft have the name recognition. And they have the active users (accounts created, apps installed, payment methods on file). But SDC is such a killer tech that whoever has it might decide it outweighs everything else and they can build their own app and get the users to come over.
If you're Uber/Lyft and this happens, you could be completely left out. Not only is your service less sexy, you're also paying labor costs that your SDC competitor isn't.