My understanding is that most of the top sports car manufacturers aren't terribly profitable. I know that Lamborghini has been passed around and around losing money most of it's life. They make lots of revenue, but their costs keep them from making a profit. The risk that monitor manufacturers would have is the same, sure they make a lot of REVENUE, off of making a 300 ppi desktop monitor, but that doesn't mean they'll make much PROFIT. Maybe someone will try it and be a success, maybe not, but it's a big risk in a field without a lot of margin left.
/. is also home to nerds who are very concerned with maximizing their numbers and having a larger e-peen. I have never said (that I remember) and will never say (unless I'm drunk or high) that no one needs super-high resolution monitors, but a lot of the people that want them only want them because the numbers are higher, not because they have a real need. It's the same thing as people complaining that their new phone doesn't have a quad-core, they don't NEED it, they probably won't benefit from it, but their computer has a quad-core so damnit their phone should too!