I believe that a lot of that comes down to the current state of affairs simply being the cheapest or most cost effective solution.

I don't know what the military is currently investigating, but if memory serves me well then the commercial industry has been long working on more silent designs (in particular regarding propellers). Of course, even there it is a constant trade-off between effectiveness (regarding propulsion) and what is considered acceptable discomfort for passengers or the environment (regarding audio).

I believe that drones could already be made way more quiet with long existing technology. The key question here might be: how much shorter do you want your flight last, and how much more expensive do you want your drone to be?

Not gliders...