If you're into gaming to such a degree that you'd spend over $1000 on a computer, then the odds are you're going to build that computer yourself. It's the only way to guarantee the quality and type of components you need.
Yes there is Alien and a few other top end manufacturers, but as most of these have been bought out by the grey box makers not too many people trust them anymore.
But the important thing is that most of us can't afford that kind of money in one go. So we gradually assemble the parts until we can build the box we want. This also enables us to replace components we compromised on as we get paid another month's wages.
Over the course of those 2 or 3 years we'll probably spend much more than if we were able to buy it all at once, but we have no choice if we don't want to find the original purchases are obsolete by the time the thing is put together.
Because we didn't buy some shite Dell or HP PC, we know that once the PC is put together, we can now replace parts as we choose. Getting the best case meant that we don't have to worry about whether a new motherboard will fit without snapping all that cheap plastic these people use to hold the insides together.
Getting the best motherboard possible means we have about 2 - 3 years of life and the ability to radically upgrade things like processor / ram combos and video cards in $200-$400 stages - unless your timing is spectacularly bad and changes to the processor pin layout make your socket type incompatible with the latest version a month after you bought it. But you'd still be better off than if you bought a grey box even if that happened so there really isn't a down side in comparison.
So yeah, I agree with the premise that the only people spending $1000 or more on a whole computer these days are those buying Macs. That's not a for or against comment, it just seems to be the case.
Another reason why people buy $400 PCs these days is they think they're getting the same thing as someone who spent big while building their own. What isn't obvious to most people is that the poor quality components in combination with the way the system is designed to be cheaply manufactured means that what appears to be a kick ass computer rarely turns out that way. It is nearly always much slower and much less of an upgrade than you hoped for, despite what appears to be a high spec on the box.