Because most gun buyers don't know any better. Many first time buyers (erroneously) assume that a new gun is going to be better and more trouble-free. More often than not nothing could be farther from the truth.
A gun dealer's highest markup comes NOT from selling new guns, but from selling used guns, instead. If you like guns, there are few occasions sweeter in life than walking into a gun shop that's just taken in a passel of guns from somebody's grieving widow! The experience is tantamount to discovering GOLD!
I've watched gunshop owners become almost orgasmic the moment they see one of these, 'gun shop widows' walks through the front door. While I don't know the percentages, I suspect that people have far less trouble with someone else's old gun THAT WORKS than with many new guns that don't.
Glock pistols immediately come to mind. Why? Because Glock has been using the ingenuous American public to field test their plastic experiments and marginally defective, 'factory-reconditioned' police trade-ins for many years, now, and always at the public's own expense. In fact, if I weren't already an armorer I wouldn't touch a Glock at all.
Over the past two decades, all of the major gun manufacturers have released their own, 'dog models' on the market. It's just another one of the risks a gun buyer takes when he purchases a new firearm. Should you buy a used gun? Learn how to check one out. If it passes muster then that's the piece you should buy first!
ADDED: Ahhh, Gregg, reread the last paragraph and stay away from older Remington 700/770's - OK. As for the presumed advantage of a new gun's warranty? Glock has the worst factory service in the world. No, wait, H&K has the worst factory service in the world. No, scratch that, Taurus has the worst factory service. Ahhh, I'm not so sure. Maybe, it's Kel-Tec that I'm thinking about. Ahh, hell, they all stink. (But Glock is, probably, the worst.)
If you want a new gun with, at least, a factory warranty that works you've got very few viable choices to make. In my opinion: Smith & Wesson; Strum-Ruger; Arsenal, Inc; Springfield; and, perhaps, Kimber lead the rest of the entire industry.
(At the present time I'm waiting to see what happens with FN? Only problem is, I've never had an FN firearm that did not work! FN seems to be a different kind of gun company.)