Aside from marketing to fools, why would they? The iPhone 5 series already has more pixels than you can literally see. I get the higher density on larger models to line up with making 2X and 3X the current display resolution. But if these phone makers just start throwing more pixels on there, WHY? It serves zero functional purpose. In fact, it is a drawback on battery life because you have to push out the pixels using the same battery.
Alas, I'm sure Samsung or someone will do it because they're out of useful ideas. I mean they did make a phone with a damn stylus.
It's been proven that even on a 4-5" device higher then 326 PPI is noticeable. One merely needs to compare to other devices on the market and you can clearly see a difference.
Don't be so easily manipulated by Apples marketing. Like the stylus, albeit it's a fairly niche market the Note series with Wacom digitizer and stylus is really cool. Just because Steve Jobs went on about how you don't need a stylus doesn't make it fact. We now see the things that are possible with it and are impossible with an iOS device.
You'll eventually see 4K on an iOS device screen but it won't be until the screen tech is cheap enough to do so. This is typically how Apple works, they do not invest into standards they wait till it's dirt cheap then implement it. They do invest into proprietary hardware though.