This is also a terrible way to run a package build system in this day and age as well, if you like. I feel exactly the same way about it, and when I wrote that I understood what it was, so I didn't need that helpful correction (I first used the FreeBSD ports system sometime around the turn of the millennia).
It's not, AUR is more like GitHub, anyone can upload content there, not like a proper repository where things are reviewed, verified and cared for.
You're complaining about "curl https://random-website.com | bash" being "a semantic detail" while it's a major difference in how much trust you can put into it. If you don't trust random-website.com, you shouldn't trust AUR packages. But very different from BSD Ports or Arch's official repositories.
GitHub also actually protects against repojacking and tombstones username/reponame combinations (that exceed a certain minimum popularity) and never lets anyone ever use them again.
The utility of AUR is also really based around being able to reuse the same repo without having to re-vet every single time. This kind of attack, that forces you to re-vet on every single upgrade so that trust inherently can't be established, is also not GitHub's model at all.
And go has a software package manager that heavily uses GH for distribution, and is arguably more VCS decentralized, but isn't vulnerable to this kind of attack, because it inherts GH's threat model, and doesn't implement the kind of choices that AUR decided to deliberately build into their system.
What does the 'R' in AUR stand for? Rutabaga?