People who shoot stock tend to shoot images they think will sell in the mass market, which tends to use those images, which leads photographers to...

Kind of a circular pattern.

But really, there's two sides to the equation. Getting more photographers to shoot and submit more inclusive images... and getting those images (and existing images) tagged correctly so that they can be found.

Did a search for "man changing clothes" and found several up front that could have served your requirements.

Same for "man behind". Should also note that in that case while unspalsh found a few suitable images, almost all of the iStock images seemed more appropriate.

So either iStock is getting more of the "niche" photos, or they're simply better at tagging. Or maybe just the photographers who submit and tag their own photos to Unsplash simply don't consider alternative tags.