That's where biology becomes problematic. From a convenience perspective, it'd be great to have a small pack of sensors somewhere that let you monitor vitals, especially for people who are sick. The problem is, there's no one place we can put a range of sensors and have them all be accurate enough to be useful. The body is a distributed system and different parts let you measure some things and not others.
Theranos ran into this problem recently by attempting to perform a wide array of tests on a single drop of blood while ignoring the basic biology behind blood-based measurements. Many of the measurements they claimed to be able to make are for things that occur in low copy-numbers in blood, which is why large draws are required to accurately measure them. If something only occurs once for every 10M blood cells and you need at least 10 copies to accurately detect it, you'll need at least 100M cells to have a chance at detection. The same is true for most other biological measurements. Sample size and location both matter.