i think if you had a burnished or brushed carbon steel gun, you'd see the same thing. A very shallow surface finish, effected by tiny little glass beads is going to end up brightly polished after being pushed into a holster a couple dozen times.
I think the real issue that most current military arms were adopted long before stainless was a viable material. S&W didn't really have stainless autos figured out until the early 80's right? I was a wee lad then, but S&W also seemed to be the first to be commercially successful with it too. Prior to that, you needed special greases to prevent galling, and all kinds of other stuff.
M-16 = 1955
M-240 = 1958
M249 = late 70's
AUG = 1970's
SA-80 = 70s
AK family = 40s
M1911 = 1911
Hipower = 1935
Glock = 80s
Sig P226 = 80s
I think you'll find that the current steel alloys are right there with the common carbon and chrome-moly steels, with a slight edge in toughness, and a big edge in erosion resistance.