As most aftermarket finishes cost between $150 and $300, how is the finish of a new gun not an important consideration?
It is a consideration. But just how important?
You can fix the finish for, as you pointed out, $150 to $300. If you can't comfortably reach the slide mounted safety/decocker on a S&W or Beretta, how much would THAT cost to fix? Lots more than $150 to $300, wouldn't you agree?
If you really dislike the relatively long trigger reset of a Kahr, how much would THAT cost to fix? Lots more than $150 to $300, wouldn't you agree?
If you hate the rear-most location of the Sig slide-stop lever, how much would THAT cost to fix? Lots more than $150 to $300, wouldn't you agree?
If you hate the location of the HK USP's mag release, how much would THAT cost to fix? Lots more than $150 to $300, wouldn't you agree?
If you hate the Sig 232's heel mag release, how much would THAT cost to fix? Lots more than $150 to $300, wouldn't you agree?
If you are not comfortable carrying a striker-fired Glock, how much would THAT cost to fix?
To me, the most important things in a gun are 1) the action type (SA, DA, striker-fired, etc.), 2) how it fits my hand, 3) the trigger feel, 4) the location of the slide stop, 5) the location of the magazine release. The weight of the trigger pull can be often be improved for relatively short money. The sights can be improved for relatively short money. The finish can be improved as well, though not as cheaply as the trigger weight. But it is relatively impractical to change most of the rest. Therefore, if a gun passes points 1 through 5 above, I can fix the finish if it isn't up to snuff. Do I factor the cost of the finish into the gun? Sure. But it isn't a deal breaker. If I can't comfortably reach the trigger, now that IS a deal breaker.
From your tag, one might assume that you are comfortable throwing hundreds at a brand new gun to make it "serviceable", but many people feel that a weapon should be complete when it leaves the factory (myself included).
Most recently, I "threw" $100 at an HK USPc trying to improve the trigger. Didn't do much. I've "thrown" $150 at a Glock trying to improve the trigger. It wasn't as bad as before, but you know what? It still sucks. Not ONLY were those guns not "complete" when they left the factory, but I can't make them "complete" even AFTER "throwing" money at them!
There are lots of things about pistols that can not practically be fixed. Finishes, as even you admit, can be readily fixed.