One feature of the system for enforcing parking ordinances, is that if your car is illegally parked, you're on the hook. Your license plate is tied to your identity, and you will be found. Even rentals or shares could have a way to identify the most recent user, otherwise the rental company has to eat the fine. With some random bike or scooter, not so easy, if there is not some kind of enforced numbering system.
I'm also guessing that with things such as bikes that are cheap enough (e.g., bikes and scooters), a growth strategy for a start-up is to address availability and ease of use first, and deal with cleaning up their mess later. Move fast and break things.
I'm an avid cyclist and cycling advocate, so I'd love to see bike sharing thrive, but I'm not brimming with great ideas.