Most who build or modify the guns do so to obtain features they want. The greatest motivators seem to be economics, challenge to do so, they don't see configurations they like, or like me, they've been doing mechanical things since they were two years old and can handle the understanding and implementation.
I remember when Kimber first marketed their 1911 - the Classic. It put the 1911 industry on its ear. I fiend'd for one. But I wouldn't buy it because I knew I would just have to do something to it...and it didn't need it. I eventually did get a Kimber, beaute of a Series I Gold Match. It wasn't quite what I wanted so sold it and acquired a SA Trophy Match. That gun is ladden with features. Sold it because I had too many on that platform. Shoulda kept it.
Some go to gun shows and such looking for guns and ammo. I go to look for parts. I means whole guns are cool, we all know that, and good buy on an appealing arm does come about, but to score a good slide, couple of barrels, real GI parts...now that's cool.
Of the 1911's I own now, only three are shadows of their original form. I've built all the others and they run well, look good, accurate and they fit and suit me. You can definitely see a progression from the earliest build to the latest. Mostly in the aesthetics. Takes a while to understand ergonomics and appealing form.
Do I save money? Yes, but only if you look at final product. My guns built to the grade they are at will cost $1600 to $2000. Okay, I don't use the high tech finishes but the parts are good and fitting is sound. But the parts I acquire eventually are close to $1k in value. I have virtually all parts for the next build. If I marketed those as the new untouched parts they are, I am looking at about $1000 income. The complete pistol would eventually sell for more than that. Inflation, a good target and certificate from a gunsmith will help seal the deal. It will not bring Wilson Combat prices because I am a nobody in that arena. But its the same quality and I build them to keep.
Sometimes one may acquire enough Frankenparts to build a beater or shooter. Properly done (i won't build unsafe or substandard), , I shoot them awhile, acquire new learning, many times in just what formats work and are desired. I sell them and I make a few $$$ when I sell it within a know market level. These are usually to guys looking for beaters or shooters.
Sorry for rambling...but this subject can bring a high when you are into it.
Here is my latest build. Still need to do the permanent finish...it will be parked. That's a full group shot off hand at 25 yards with ball ammo. It will do it all day long. Well, on shot is covered up by the quarter...but I called that. Got excited.