why would you rent one when you could own one?

Because storing them and dealing with them is a massive pain. I use these scooters way more than I use my bike, and it's not because I don't want to pedal.

When I go out on a scooter I like the flexibility of leaving it at the first place I go. This prevents me from needing to leave my bike parked overnight if I decide to go somewhere else that isn't bike friendly. Not to mention that stealing scooters is really easy and bringing them inside most places isn't really an option.