Sometimes, when I'm using a public restroom and someone is in the stall next to mine, I start obsessing on the fact that the side of my naked ass is eighteen inches away (tops!) from the side-ass of a total stranger. The presence of the stall wall makes this proximity tolerable, but nonetheless I'm technically performing eliminatory functions way closer to an unknown person than I would ever choose to.
Imagine: the difference between "brief and totally unremarkable pit stop" and "lifelong trauma and humiliation" is just that thin little wall that's not soundproof and doesn't even go all the way to the floor. We laugh at parakeets for thinking it's nighttime when you cover their cage, but how different is this, really? "Well, I can't see other people - except for their feet - so clearly this is a completely safe and secluded place."
By the way...I'm tall, and most bathroom stalls only come up to my nose. Which isn't a big deal for me - I'm sitting down once I get in there anyway, so I can still maintain some small illusion of privacy. But the walls of fitting rooms are usually that same height, and changing one's clothes is a standing-up activity. Have you ever accidentally made eye contact with a stranger while you were semi-naked? It's...awkward. Especially when you're in a highly undignified struggle with a garment that turned out to be too small.