why do expensive amplifiers produce a better soundstage

I would suggest that with “more expensive amps” , one has also heard that amp connected to “more expensive” components, speakers and cables, and likely in a room that has had some work go into it, at least into seating and speaker placement. That all matters. I would suggest that if one were to do an A/B test with a “less expensive” amp and an “expensive” amp with all other factors the same, and both amps had the same power ratings equally driving the speakers well that no one would be able to discern the difference in a blind listening test.  

Why do expensive amplifiers often produce a better soundstage?

They don’t create soundstage — they preserve spatial information that cheaper amps partially destroy.

Soundstage depends on extremely small timing and phase cues between channels, plus very low-level ambient information (reverb tails, decay, hall reflections). Less expensive amplifiers often blur these cues due to phase shift across frequency, power-supply modulation under real speaker loads, higher noise modulation at low signal levels, and reduced channel separation when driving reactive speakers.

Higher-end amplifiers typically have:

• Better phase integrity and time-domain behavior
• Lower noise and better low-level linearity
• Much stiffer power supplies (less rail sag and micro-compression)
• Better channel isolation and grounding under load

The result is not “added” width or depth, but less smearing, so images stay stable, depth increases, and the stage feels more continuous and believable.

In short:

The soundstage is already on the recording. Better amplifiers simply get out of the way.

It’s amazing that this issue seems to come up over and over again.

If you had the experience, you'd find that it's not actually the case.

Share it. This is what the forum is all about.  Maybe you could give AI and everyone here a good lessen what the soundstage feels like.

I think one of the most important factors is the quality of the recording because no matter how good your system is the recording is what it is.

Further, compression of the recording and detail available to transmit through the room via pressure amplitudes is really important to overall soundstage. The density of the pressure amplitudes and it's impact on your inner eardrum is ultimately what it's all about.

It's kind of like porn you know it when you hear it. It's not rocket science

If you had the experience, you'd find that it's not actually the case.

Share it. This is what the forum is all about.  Maybe you could give AI and everyone here a good lessen what actual soundstage should feel like.