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.

A good read on this topic is The Pursuit of Noninvasive Glucose: 'Hunting the Deceitful Turkey'.

The biggest problem is that they are racist. Optical blood sugar sensing devices were developed a decade ago. They were pretty accurate for pale white people, ballpark for most skin tones and don’t work at all for Black people. Good luck bringing that to market. Apple threw out the whole product and restarted with infrared.