These studies cost tens or hundreds of millions of dollars, take months or years to run, and then the process of getting the FDA to approve your product takes more millions of dollars and more years. At the end you have approval for a particular product for a particular purpose, and if you want to launch v2.0 or to use the product's measurements in a new kind of analysis for a new disease, you need to go through the approval process again.

This is entirely incompatible with quickly evolving technology in a market with many small / new players trying out various ideas. You end up with only a company the size of Apple being able to launch an Apple Watch, and even then it only being able to tell you your heartrate.

If the Apple Watch had advanced sensors for e.g. blood spectroscopy, and people could write programs that would tell you your blood sugar rates, and other people would write apps to tell if you had diabetes or advise you on insulin dosage, and people could side-load these apps without going through the Apple Store (maybe Android would be more appropriate), do you think the FDA wouldn't step in?