The same reason you don’t carry a camera, a music player, a phone, etc as separate devices in your pocket. Because it’s wildly inconvenient and super frustrating to swap between them. For diabetics in this case, you generally have two separate companies making the pump and the glucose monitor. So at that point you are carrying a phone around, a monitor for your glucose levels, and a controller for your pump. That’s three devices that you need to keep charged and on your person at all times. Not to mention they are generally not slim and sleek and easy to pocket.

The ability to swap between these from a single device and the mental offload that brings can’t be overstated.

That being said, people that use medical services on their phones should not do OS upgrades until they are notified by their makers to be verified and working and should be heavily tested before any updates go out.