Cost of machining and volume production has pushed a lot of makers into using extrusions. They are nearly finish dimensions in two directions with a minimal amount left to cut.
Rails are actually an embarrassment in the AR market - they are just an accessory mount, and frequently are unused. Many cost far more than the accessory attached to them, and to a significant degree, the holes milled in them are largely decorative beyond those that facilitate attachment.
Most sell for style - not function. The M4 quad rail? An institutional compromise, heavy, redundant, with four feet of rail, and not user friendly, hence the covers. Troops generally wear gloves anyway because the environment is rough, the rails only add more abrasion.
I see no reason that someone couldn't extrude a nice stiff tube and then overmold a polymer covering, leaving only the exposed mounting holes. And market it for under $75. Oh, wait - Hogue has been doing that for how long?