It's because that's what the equipment is designed to use, and yes it provides more reliable results with the current technology. Much of the equipment used in healthcare relies on very old technology. This certainly does not mean it cannot be improved to work with less amount of material.

There are hand held sequencers for your phone now (minIon) that are very cheap and work. There are also large multimillion dollar bench top machines on the same technology from nanopore. Both are very new technology compared to what we used in early 2000s, and offer various different features and certainties from the analysis.

However, it is foolish to say that the minIon is impossible. It works. It's just less resolute than other systems.

This is true for many systems which people cry "Theranos" about. Their issue was lying. That was the problem.