We know how to accurately test the blood for various things if we can actually obtain a sample of the blood itself. This is straightforward, but you have to puncture the skin to get the blood.
We are starting to be able to infer/guess the blood content of other things, e.g. oxygen, with "useful" accuracy, by measuring externally-detectable changes, such as the color of blood vessels when you shine lights through the skin.
But nobody has ever (so far) been able to do this with glucose. It may be possible one day, but we don't know that it is. Even with sensors 1000x or 10000x more precise than we have today, the glucose content of blood still might be just be one of those things that just doesn't manifest enough external changes to be detected non-invasively. Thus, we still might not be able to do it 100 years from now.
(I hope we can; I know people with diabetes and I also just want my watch to give me this data about myself because, I mean, why not?)