To answer your question of "why don't you see them," I think it comes down to power requirements.
As mentioned, new-ish first/business class generally will have outlets available. You can explore different aircraft here.
The reason it's not available in every seat is capacity. If you have 150 people pulling 75W, that's 11.25 kW. On a 737 NG, each of the two generators is rated at 90KVA. However, they need most of that power to run the radio equipment, avionics, lights, galleys, window heat, entertainment system, and everything else on the plane that runs on electrical power. So besides a possible upgrade of the generators/power system, this also adds the weight of running cables and having outlets at every seat. It's a lot easier to do that sort of thing on a train car, where it doesn't have to fly and can get power from the locomotive.
You can find more about the 737 electric system here.