Scripts in bin have no documentation, no easy way to enumerate them, etc. There is definitely a time and a place for bin scripts, especially as things grow in complexity. However the beauty of just is that there's one file (the justfile) that defines all of your project's actions. You don't have to go spelunking into bin to figure out how to tweak a compiler flag, etc. And since just will run anything there's no reason why your complex bin scripts can't just be called from a simple one liner task in a justfile.
Could your write a bash script that does stuff like enumerate all the bin scripts, pull out documentation comments, etc.? Absolutely, and people have followed that pattern for a while (see https://github.com/qrush/sub) but it's a bunch of boilerplate to copy between projects. Just pulls out that logic into a simpler config file.
I've thought about trying it out a few times but can never see its value over scripts in ./bin.