1920x1200 monitors used to cost about $400 around 2005.
2560x1600 monitors now cost about $700 (used) or $1100-1300 (new).
Where to find
To find some high resolution monitors:
- do a Google product search for "monitor" and filter by 2560x1600 (or desired) resolution
- search Amazon by typing in desired resolution, may need to negatively filter some monitors out
Maximum graphics card resolution as a cause of resolution ceiling
2560x1600 is the current "maximum" of consumer-level hardware as of 2011 or so, since that is the maximum resolution of most graphics cards (you can check this yourself by looking at the specs of many consumer/professional graphics cards; I haven't checked the ATI cards as thoroughly as NVidia though). Higher resolution would require either a specialized graphics card, or quad-display configuration with two graphics cards and perhaps an adapter; I am not entirely sure about the specifics.
Reasons 1920x1200 may be the highest resolution produced
The reasons monitors probably tail off around 1920x1200 are economic reasons, due to consumers who don't know the difference between screen size and resolution, economies of scale, push to 120Hz "3d" (alternating-shutter-glasses) using up bandwidth, new LCD technologies starting off small, etc. However there may also be a minor technical reason that might possibly contribute, e.g. normal DVI supports at most WUXGA (1920x1200) at 60 Hz, and one requires "dual link" DVI (a modified version of a DVI cable with extra pins) to get higher resolutions/refresh rates.
Or as you say, it could be a conspiracy of price fixation, as another answer suggests. There are times that I have jokingly wondered that myself, with the trend in decreased monitor resolution, especially of laptops.
1920x1050 is also "Full HDTV"... and we know the power of marketing.
Special high-resolution monitors
There are a few rare monitors (e.g. IBM T220/T221 ~22" monitors) with extremely high resolution, but the DPI is so high that the pixels will blur together if you don't have 20-20 vision.
If one enters the realm of medical imaging or projectors, it is possible to achieve extremely high resolutions, but you have to be a millionaire (or company) to afford them.
Why resolution isn't the whole story
Even then, resolution isn't the whole story of a monitor, since there are issues of latency. Some monitors (like the T220s) cannot play movies or drag windows because the rise-fall latency of the pixels leads to ghosting. Many high-resolution monitors may suffer from major issues (like pink-blue areas on the 30" Dells). One may need to buy a stand to adjust the height. If someone is reading this and choosing a monitor, I would encourage them to pay attention to the details.