Small appliances rectify AC into DC before using it and therefore what you end up with is a DC supply that can support multiple input voltages and frequencies.

By contrast, larger appliances typically use the AC directly. A motor is designed to be able to connect directly to local AC power, and the voltage is quite important and so is the frequency, because a change from 50 hertz to 60 hertz will make a motor go much faster and a change from 60 hertz to 50 hertz will make a motor go much slower, and resistive loads such as furnace elements which are designed to be able to be connected directly to mains power our engineered with a specific resistance.

For motors changing frequencies, the way you would end up having to retrofit such devices is to rectify the DC signal and then reconstruct it as the proper voltage and frequency using pulse with modulation. That's going to be a fairly expensive thing compared to just connecting some wires across mains. Even changing voltage is using a transformer means that you're going to need transformer capable of potentially converting multiple kilowatts of power, and that's not an inexpensive touch either.