Invasive CGMs (Continuous Glucose Monitor) and minimally invasive CGMs exist. It's the non-invasive CGMs which are being worked on because managing an implant or having something always poking through you is a big annoyance+risk compared to wearing a watch, ring, or patch.
The problems for these non-invasive options so far is they either too easily fail to work, are bulky/expensive, give very ballpark accuracies, or some combination thereof.