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

Naim Audio uses seperate power supplies for there amplifiers, preamplifiers, streaming devices. Many units can be fitted with two separate power supplies, one for each channel. The Supernait 3 Integrated Amplifier can also be upgraded by using their HiCap outboard power supply to power the preamp section. Naim recommends using two Naim Frame Audio Racks to separate the large power supplies on one rack and the critical circuitry of the amplifier, preamplifier and streaming devices on the other. Naim’s philosophy - separate the Brian’s from the Braun. This technique keeps any transformer hum from compromising the delicate Audio signal.

Naim do.

PS 3000 HV Power supply (ta-hifi.de)

My amp has two power supplies and a battery.

NAIM I haven’t heard much about this brand, anybody know if it’s any good?