Because it costs money and generally they aren't needed. A 30 amp outlet is plenty to start a single air/con and a 50amp outlet is plenty to start 2 air/con units (or even 3 units).
It's really only when you try to use an undersized generator or overload a 30amp outlet, where these units really become useful.
Also running 2 air/con units is pretty marginal on a 30amp outlet. Keep in mind, 30amps is the nominal rating. Continuous output is limited to 80% or 24amps. You see a lot of burnt up 30amp plugs from people who think they have 30amps, so they are going to use 30 amps.
