AskHistorians

You've gotten answers pretty much covering all the reasons. I want to comment about Roger's Rangers, which is an example to show that military leaders in the 1700s were not ignorant of the value of camouflaged, stealthy units.

This was a British light infantry unit, starting as a single company and eventually expanded to a corps, that was first active in the French & Indian War. This unit was valued for its ability to scout, and perform unique missions like raids and ambushes deep in enemy territory.

The unit was active on the side of the Crown in the American Revolution, though some veterans joined the continental forces, meaning both sides used light infantry scouts.

Ranging units were a known and valued asset, however a military entirely made up of light infantry rangers wouldn't function. While these units could ambush and raid with the element of surprise on their side, in open field battles or static sieges they lost a lot of their punch. These units could deal damage and gather intelligence, but couldn't be relied on to hold ground against a conventional military assault.