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

Like these Valvet?

The only reason not to is cost. The reason to is mains isolation. The distance between them is irrelevant provided a shielded cable of suitable gauge is provided. Yes shielded as DC is easily contaminated.

If you don't have separates, and you're brave enough, open the case and with a sheet of aluminium, cut and fit shielding around the power transformer and if possible around the DC power board. Don't stop there, do all your equipment and you'll be shocked at the improvement, no matter what the purchase price.

I design power amps.

Simple: cost.  The added chassis adds a LOT of cost that could be invested in more important improvements. And it hits again raising shipping costs, also a LOT.  The advantages can be minimized through shielding of the only noisy parts - the transformers. Finally, all power amp signals are fairly large, so they are not terribly suceptible to noise compared to things like DACs and RIAA (phono) stages.  The input to a power amp is, by convention ~ 1V rmp for full output.  The input of some moving coil cartridges, by way of comparison, is 1/2 of 1/1000th V (in other words 00.5 mV). Line stage preamps fall int he middle.

That said, for a recent prototype that i constructed i did just that - mostly to make it easier to work on and experiment with tho.....but everyone seems to thinkl its cool. :-)

I would say to cost and plus designing a psu takes time and effort that people don’t want to use. Also home equipment is a copy from pro equipment and all pro equipment is made to fit in a standard 19 inch rack so since home copies they use very similar designed circuitry if not an exact copy just they don’t copy the chassis and standard 19 inch rack design. Also I know with professional amps crown xli and xls the power section is shielded fairly decently so putting a separate power block doesn’t make much sense. If you want separate power blocks I do believe Bursonaudio has them for sale and they aren’t so cheap a couple of thousand for a decent one so enjoy and know this if you listen to mostly music pro is the way to go. Kudos

They do not just copy pro equipment.