Ever seen a Colt Government 1911A1 customized? When they sold for $25 lots of gunsmiths made a living at tweaking them and selling "comp race guns." There is a market for you and me where we can buy one of these, finish the interior shaping, then fit the gun to our ideas of how it should handle and look. You can build a Ruger 10/22 with nothing more than the serial numbered receiver, all other parts custom, same as an AR15. No two finished guns will look alike.
It's really no different than sporterizing all those milsurp rifles left over after WWII. And in case we missed it - opened up a copy of Rod and Custom magazine lately? Nobody builds a factory stock 1932 Ford Coupe any more.
On the other foot, there are some things we simply cannot afford due to rarity and limited supply. So - it's a replica kit or nothing at all. Plenty of kit Cobras running around, in fact, if you see a Cobra on the road - a 1965 427 Shelby Cobra - you are likely seeing a completed kit with about zero original parts. Tens of thousands of kits exist, only a few hundred originals were built.
Are their little glitches or some functional issues cropping up? You bet. However, the owners live with them as that is the whole point, how to figure out building the contraption and make it run better than stock. In some cases we deliberately trade off performance in one area to enhance it in another, take a look at a open class 1911 race gun and you will see it's nearly impossible to reasonably use one as a CCW.
So, just like a 32 Ford, if you want to chop and channel a Glock or 1911 or AR15 or Mauser 98K go right ahead, tens of thousands of others do and have been since the 1950's of recent note. They even hot rod LED flashlights, ink pens, and computers. Ask your techy buddy about Ubuntu on a laptop. There is a whole world of folks modding stuff because they don't see factory stock as all that, and in some cases, it's actually an improvement.
Now and again.