If you're type 1 diabetic, accuracy is paramount, especially in a closed loop insulin pump setup. Even with access to the bloodstream, existing CGMs leave plenty of room for improvement that would directly improve quality of life a lot more than the annoyance of applying the sensor under the skin.
> The existing alternatives do have access to the bloodstream.
So? Who says otherwise? People don't want invasive tests that have direct contact with their bloodstream.