It's not just marketing. People were trying to make scope mounts for SKS's and AK's in their garages before anyone tried to market those things.
Matching a military configuration is irrelevant for many of the uses those rifles see. Yes, it's cool to have it close to the original configuration used in actual combat. But it can be fun to have it in a million other ways that are possible.
I am keeping my SKS as close to stock as I can. It feels right to me. If someone else bubba's theirs to no end I couldn't care less. I'll probably still make fun of them, though.
Shorty M14s may not have seen real service but they are handy for shlubs like me who just want to have some fun and maybe hunt a buck or too.
I do admit that the 22" barrel M14 type rifle does look better than an 18.5" one. Tempting, for sure.