Dark_art_: Well, nostalgia may be a big factor but one good reason might be the use of CRT monitors, wich some people still prefer to modern displays.
That's actually a good point. I think emulation is good enough in all other aspects, but a CRT had a way to charmingly make lower resolutions look good, while the merciless sharpness of modern panels exposes all the ugly pixels.
That's also a reason I kind of dislike retro-pixel look. I simply looked different, less "in your face" pixellated back in the days, because of CRT.
I think it won't be long until that is caught up with though. The current HD displays are too sharp to do it natively, but not sharp enough to emulate the comforting "adapting unsharpness" of CRT, but 4k is already another matter. And it won't be long until clever emu-devs find ways to make it look like the real thing on those screens.
Another thing that can't be emulated, and that's something that won't fix, is the haptic experience of the joysticks of the time. Because that hardware isn't made any more, and even if you manage to get something like that, and make it work with adapters and what not, you wouldn't dare to mistreat it like in ye olde days.... so many I broke... ;-)
WinterSnowfall: There's also the distinctive sound of floppy disks and tape loaders, though I've heard some emulators will even play similar sounds during load times to recreate that retro atmosphere :).
Some emus do it. Some even emulate the original loading times. Pretty Zen were were back then....