Why are headphone amps so expensive?

[Rubycon]

That's the nice thing about headphones, earspeakers, in ear monitors, cans, or whatever you choose to call them. Portability! An amp that sounds very good on one pair may not on another - impedance and listening preferences vary wildly so the best test is your own ears, program and cans.

This is something you cannot do with speakers - even if you could bring your speakers into the store their listening room is completely different than yours.