They do. There's a list of them on the Arch Wiki:

https://wiki.archlinux.org/title/Unofficial_user_repositorie...

The problem with user repos vs the AUR is that you're trusting the maintainer behind them instead of inspecting the PKGBUILD and fetched sources yourself.

And they're incredibly unpopular.

The entire problem is that people weren't inspecting PKGBUILDs. At least with user repositories, you have someone who can build a reputation for trustworthy packages. There's a reason why every other distro does user repos.