Why don't higher end amplifiers come with a separate power chassis?

There are a few, but amplifiers aren’t usually as sensitive as preamps, so there really isn’t much nee for separate power supplies.

Pass labs does it in their XS series.

It's very expensive and as stated above not as necessary as in a preamp with very low level signals.

It comes down to the ratio of transformer noise to output signal: high on preamp, low on power amp.

In addition to the examples cited above, Border Patrol Audio uses separate chassis for signal path and power supply. Thomas Mayer (Germany) employs this approach with some of his amplifier models. I can see a strong case to be made for either this method and the more common/traditional mono block design.

Charles