why do expensive amplifiers produce a better soundstage

I think it has to do with the more expensive amplifiers ability to reproduce fine details, which are required to provide the audible cues that are required for soundstage.  Phase, timing, crosstalk, rolloff . . .  and the like all help.  Everytime I take a step up in equipment that increases details and noise reduction, the soundstage improves, but not necessarily the bass or appeal of the treble.  I agree with others though, speaker placement and the room mater the most.  

Can an amplifier interfere with sound staging? Yes. Can an amplifier create it? No, not on its own. Sound staging is all about timing and phase, so your source, speakers, and the acoustics of your room will have a much greater effect than your amp. Enjoy!

Probably a function of current output capacity.

More expensive components may help, but I don’t know why

Thank you to everyone who who took the time to answer my question and Happy New Year!

@dpac996  Thanks for suggesting Doug Self