Why are most High End Amps class A

Most of you are thinking of 1 type of class-A power amp - the type that Krell, Plinius, Threshold, Forte, Sugden have made/are making. This type of class-A power amp does dissipate a lot of heat as it is biased to run full power bias current all the time. The heatsinks on these amps are large (they have to be!) & the current draw is high thereby running up your electric bill.
There is another category of class-A amps that use sliding bias. I believe that Symphonic Line & Karan amps (there might be others but I do not know) fall into this category. These amps run cool when there is no signal or when the signal is low (from my experience I would say when the SPL level at the listener's chair ~9-10' away is in the 80dB range). As you crank up the signal input, the dissipation increases markedly. These amps do not run up your electric bill.
Just FYI.

As others have pointed out - a well executed class-AB amp can be very satisfying sonically. I can say this from my own personal experience.
However, [b]THERE IS[/b] a certain sweetness to the sound from a class-A amp. I realize that more when I take out the class-A amp & put back the class-AB amp. It's much easier for me to discern what I lost (when I put the class-AB in) than what I gained (when I put the class-A in). In my particular case I say "lost" because I personally like the sweetness of the class-A sonics. YMMV.

It would be interesting to see how the Symphonic Line ($$$$) vs. a Plinius ($$) would sound side by side. Manual vs automatic transmission.

I wonder if all high bias current amps, or the sliding bias current high current as needed is where that extra sweet audiophile sound is coming from. I am sure it is just part of the mix, but the Plinius looks like one of the least expensive SS choices to get 100 watts of class A power.

When did audiophile's start to worry about the electric bill
There are some great A/B Amps but they are not great because they are less expensive to operate or that they cost less to obtain.
To think that way is to say a Honda is better then a Ferrari
because it gets better gas mileage.
I find Class A amps to sound better then the Class A/B or D that I have heard.
This hobby is about SOUND nothing more and nothing less.

Bombaywalla - not only bias but also gain (before feedback)is different between class A and AB. Class AB requires much higher gain (usually about 10x) to linearize output transistors with deeper negative feedback. Large gain causes amplifier to overshoot (and even choke) on fast changing input (pulse) since it is unable to feed signal back in the same phase (delays the signal). This is called Transient Intermodulation Distortion and was dicovered in 1970. Before that first SS amplifiers had excellent THD and IMD but unpleasant sound (overshooting introduces odd harmonics).

Using different classes of amps for woofer and tweeter is fine if we can guarantee that phase shift is the same.

Most amps are NOT class A, but AB