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.
Doing that with your own one has downsides as you have to carry it along all the time.
- You know when you pick it up that it's going to function well, because you maintain it
- You know it isn't going to have sticky handlebars because the last person to use it was eating a candy bar
- You know it'll be charged, assuming you keep your devices charged