Here's your problem, simply stated: You are unable to understand the evolution of the Continental Army.
Again:
From this non-coordinated array of various counties’ and colonies’ amateurish militia companies encircling Boston during the summer of 1775 would develop, within four years, an army equal in professionalism and esprit with that of any major European power.
The militia BECAME an army equal in professionalism and esprit with that of any major European power.
At the start they were not an 18th Century professional standing army.
They were a militia that knew how to fight the fights they had to fight, primarily "Indian wars".
If you had been a skilled Indian fighter at the time, you'd have thought it absolute lunacy to stand in a line, take cannon fire and then take volley fire from massed enemy. I wonder how many ran from fear as opposed to how many ran from correctly figuring out that there was a better way to fight.
As the Brits retreated from Lexington and Concord, the militia fought "their fight". They fought as Indian fighters. How'd that turn out for the Brits? How many militia ran then?