Why can't you buy a monitor with a resolution over 2560 x 1600?

The larger the screen the harder it is to make, and thus the more it costs to produce each unit (obviously) and the lower yields you get.

It is not cost effective to make extremely high resolution displays as they would cost several times what the largest displays currently cost. When they improve the manufacturing process enough that they figure they can make money by going to a higher resolution they will. Until that time they simply would not be able to make a profit without selling the displays at absolutely impossible prices, which no one would buy.. thus they would not make any money anyway.

No one would buy a $20,000 display instead of a $1,000 display just to get a 25% improvement on pixel pitch. AT least not enough people would buy one to warrant the cost of production.

When OLED's start becoming more common you will see larger screens being produces with similar pixel pitch to smaller ones as OLED's will be far easier to mass produce once all the bugs are worked out.

If you are rich enough I'm sure you could talk to sony and have them produce a custom display for you. I'm sure it is done all the time for folks with multi million dollar tv rooms