I'm not sure what you mean. If they're applying for paid developer positions, they would need a work visa anyway. I've only ever hired for remote freelancer positions, so I've never been involved with any of my contractors' visa details.

Why would this exclude people with family obligations? When I say "narrowly-acoped project," I mean 5-10 hours of work. I've never gotten an offer for a dev role where the application interview process took less than 5 hours, and those were unpaid hours.

Most US work visas are limited to a single specific employer. Some can be transferred to another employer, once that employer extends an offer - but this transfer has costs in processing time & fees.

In other words, some visa holders in the US are very much allowed to work, but not contract work.

Typically these are candidates, most of whom are already working right? So their visa would be tied to their current employer and they are not allowed to work for someone else.

5-10 hours would be similar to a typical interview loop yes, and if that was the entirety of it then it's not more time consuming than the typical engineering interview.