I think Apple has better QA with their displays, and they probably choose the cream of the crop.I agree people act like there is a big defect and just like with most things it is probably such a small, minority of the purchased displays.
However, in the wider display market, it's FULL of problematic displays. I've actually bought 4 displays in the past month, and none of them are as good as my 5K iMac's display, due to various issues.
1. One had stuck pixels, but only in one monitor setting. So it was partially firmware related, but nonetheless the stuck pixels were there when that setting was turn on. Very strange. It also had mild backlight bleed.
2. One had quite bad backlight bleed in one area.
3. One had the wrong monitor in the box. OK I can't blame the manufacturer for this one.
4. One is generally good, but has mediocre brightness uniformity. Some people don't notice this, but I notice it. I kept this one though, since otherwise it's very decent.
Despite my 5K iMac being 5.5 years old now, its screen is still visibly better than ALL of the these, even though the new screens I had purchased have more features (like HDR support). And I'm not talking about the resolution. I'm talking about other factors such as brightness uniformity and backlight bleed.