Why would they want to do this in home/pro audio amplifiers?
One reason is space. Since you are dealing with 50/60hz
transformers you need "big uns" to get some high powered amplifiers
plus "big uns" transformers are very ineffecient, alot of power
is lost to heat.

High frequency switching power supplies done correctly can
reduce size because they require a much smaller transformer
(toroidal typically), they are more effecient. The reduction
in size is because they switch the transformer on/off not
at 50/60 hz but perhaps anywhere from 20khz - 100khz depending
on who designed the circuit.

Some people may argue that switching noises may leak into the audio
signal path. Sure, if the amplifier is poorly designed. Plus,
can your ears hear this noise ?

I've done car audio for 10 years and didn't experience any leakage
noise problems, even with cheaper amplifiers.

Still not sure? go listen to one and be 100% sure.

Oh, the reason most home amplifier companies don't do this
is because of cost. It's cheaper to just buy a beefy 50 pound
transformer, add a bridge rectifier and some storage capacitors
and the power supply circuit is done and it's realiable.

Ask the same designer to convert to a high frequency switching
power supply and he will say "oh no, now I have to do some
real engineering work".