It's not like there aren't any ... take Matrox Parhelia for example. Both DVI outs on that one are dual channel. And you even have $100 left then
Whether or not some of the more mainstream graphics cards implement the double-link-ness on the DVI output, who knows ... card specifications are mostly blurry on that issue.
There are quite a few VGA chips that, besides the integrated single channel TMDS transmitter, have an alternate path to an external one that may then be dual channel capable. Knowing that, and seeing that mainstream VGA cards do not have separate transmitter chips on, it's all single channel there, it seems.
Besides, even single channel TMDS has a two-pixels-per-clock mode that doubles the pixel throughput at the expense of color depth. This is a very common thing on large TFT panels.
An additional thing that is being done right now is the migration to "reduced blanking period" timings on both ends, reducing the amount of time spent in horizontal/vertical beam flyback. This brings the pixel frequency of a given resolution and refresh rate down - current CRTs typically waste 1/3 of the signal bandwidth in the front and back porch of the actual image signal. That's quite a bit of headroom waiting to be squeezed out.
regards, Peter