That is largely what most of the pumps are, but it's basically a device that's constantly embedded in your body and drip-feeding you insulin (and a button for dumping a larger amount when you're eating). Mechanically, it is very much as simple as it gets: insulin storage, delivery system, and a little computer controlling a valve.

Generally, the spots where you can reliably measure your glucose and the places where you can stick the pump for weeks at a time are not especially close. Continuous monitoring means the pump can adjust the dosage (and shut off when you're in range) without your input, which prevents tons of complications.

The pumps already require a tube between the actual device and the needle that goes into your body, and they're compact but by no means form-fitting. Adding sensors, displays, and other functionality there means more bulk, worse quality of life, and if the sensor fails that's the whole device dead. Separate sensors mean if you brush your shoulder against a door a little too hard, you just replace the relatively cheap monitor instead of the whole system.

Phones are built to talk to shit over Bluetooth and have lots of extra space to build in fault tolerance and fancy graphs and whatever other garbage, leaving the little miracle tech alone to do one thing really well. A Bluetooth radio is really small, but the little computer that needs to live next to it to do anything with the signals adds bulk that's hard to balance with quality of life.