With all due respect.
1. Granted, large pixel count (for arguments sake, by this I mean > Full HD) are purchasable, but only for an inflated cost.
2. The manufacturers have gradually changed aspect ratios, from 4:3 -> 5:4 -> 16:10 to 16:9. There is where slick marketing has come into play. My assertion: People did not want this (source, many forum posts on just this issue). So my assertion should be prefaced with "some". These people I will also assume had some inkling of what the resolution change would mean, so lets call them "knowledgeable".
3. Marketers got to work with things like wide-screen, HD and even simple 19". Would you buy a 19" screen or a 17" screen? Bigger is better right? E-peen anyone? Of course I expect nobody here to fall into such a simple trap. Resolution, aspect ratio, etc. etc.
4. Computers have become commodity. This next is said tongue in cheek--I practice no form of elitism. Unfortunately, the intelligentsia meets the great unwashed, and there are more of them than the former. Big numbers, emotive slogans won. The manufacturers applauded because it's cheaper to make a 19" 16:9 display than a 4:3. And don't get me started on TN panels. Capitalism won out tovarishch.
And why hasn't pixel density gotten better? Because it is not in the manufacturers interest. Higher pixel densities cost more to make. So in effect, after driving out the smaller players, and then consolidation amongst the rest, we have in effect, if not actual, a cartel or oligarchy.
What you really need is a disruptive technology to come to the fore.