I think it has a lot to do with "curl|bash". Cut&paste a curl|bash command-line disables all inherent mechanisms and stumbling blocks that would ensure properly ensuring trust. It was basically invented to make it easy to install software by circumventing all protection a Linux distribution would traditionally provide. It also eliminates all possibility for independent verification about what was installed or done on the machine.
Downloading a deb via a package manager is more secure. Downloading a deb, comparing the hash (or at least noting down the hash) would also already be more secure.
But yes, that the run arbitrary scripts is also a known issue, but this is not the main point as most code you download will be run at some point (and ideally this needs sandboxing of applications to fix).
Well, distros haven't really put any effort into making it viable as far as I know. They really should! Why isn't there a standard Linux package format that all distros support? Flatpak is fine for user GUI apps but I don't think it would be feasible to e.g. distribute Rust via a Flatpak.
(And when I say fine, I haven't actually used it successfully yet.)
I think distros don't want this though. They all want everyone to use their format, and spend time uploading software into their repo. Which just means that people don't.
Downloading and installing a `.deb` or `.rpm` is going to be no more secure. They can run arbitrary scripts too.