Substances found in blood are present at microscopic scale and tend to be invisible to the unaided eye. Why is a whole vial of blood (at ml scale) is required to assess the presence and concentration of substances, given that the modern equipments and testing is becoming more and more sensitive and efficient?
It might be better to consider the sampling technology, economic and logistical issues with this question as well as the tech behind the tests.
First, some tests still will want a few milliliters of blood - e.g. cell counts for specific cell types. Then there is the need to create and stock many different kinds of sample collection devices and train the collection centers to use them effectively- the industry won't be wanting to completely retool whenever the volume requirement for the assays changes.
Mainly, since the expense of collecting and shipping a few milliliters of blood is pretty much identical to doing so with a few microliters of blood, what has happened is that the tubes are used to do multiple assays with excess included in case an assay has to be redone.
Probably all this blood currently collected is not necessary at all. I expect that the cost of changing all the blood collection systems and protocols is not worth the cost of collecting the exact minimum amount of blood needed. The benefit to the patient or the lab between collecting 3-5 ml of blood and collecting 50 microliters of blood to be shipped off for lab analysis is probably close to zero.
Having more blood than you need does not hurt the lab either should they need to repeat the assay or when you need to use the sample for more than one different test.