During my time here (a few months) I would categorize this forum as more of an offroad oriented enthusiast website. This being said, everybody wants their vehicles to serve different purposes. I know I speak for many when I say that my DD needs to accomplish a myriad of tasks and, while I would love to fill half of my cargo area with subwoofers . . . this just isn't feasible with the other purposes that my 4Runner serves. Life is all about identifying priorities and striking a balance between them.
In response to your original question: it probably has a lot to do with laziness. There are few who are willing to take the time to find the differences between quality components and overpriced crap. It could also have alot to do with the priorities of the build. If I had $10K to spend on a 4Runner build (not including the Runner) I would spend $1000 on audio. But I don't, so . . . I'm going to nickel and dime my way to more refined audio.
To every man his own.