This is pretty much exactly what you're looking for. A phone OS on the phone, plug it in, get a desktop OS on a desktop. It worked pretty well, even for the limited hardware at the time.
If a device supports Displayport Alternate Mode on USB-C, there are docks that work like how you're imagining. But I do agree, for a lot of devices the mobile OS just doesn't really have a great desktop experience.
https://www.microsoft.com/en-us/windows/continuum
This is pretty much exactly what you're looking for. A phone OS on the phone, plug it in, get a desktop OS on a desktop. It worked pretty well, even for the limited hardware at the time.
If a device supports Displayport Alternate Mode on USB-C, there are docks that work like how you're imagining. But I do agree, for a lot of devices the mobile OS just doesn't really have a great desktop experience.
https://plugable.com/products/uds-7in1