You won't see them because they will cost too much to produce (effectively 2 radios in one), and little customer demand. The only reason amateur radio transceivers usually have 2m/6m or 2m/70cm is because there is a demand for them.

CB isn't as popular as a choice for personal communications system, as say FRS is. That's not to say that CB isn't useful for the road. People have called GMRS the "new" CB, but in reality, at least where I'm at, there aren't a lot of users.

The "newer" generations aren't to much up on wireless technology, unless it's their cell phone. The closest most will get to wireless radio communication is probably Nextel's DirectConnect. Apparently they now feature a way to connect between two handsets directly off network.

edited to add: The cost of a dual-band CB/FRS radio may not be a real factor, as some CB radios do have WX channels in them (so that would be 27Mhz for the CB, and 162Mhz for the WX channels). I'm sure there is something technical I don't know. However, probably not too much of a concern to you...