GPG adds usability problems and doesn’t much help in the case where people have no idea whether the remote author they’ve never met is trustworthy. In most cases something like a Linux distribution is what you want where things are at least highly visible and a trusted third-party is looking at the update history.

Modern package managers do at least store hashes so your NPM, Python, Rust, etc. packages can depend on other packages with hashes in addition to just a version, which at least forces them to attempt to make the exploit covert enough that it can be deployed to everyone but there are many ways to make something subtly vulnerable. Ultimately, I really think this is coming back to securing the environment so a successful attack gets less. Apple has led the way on protecting things like passwords and mail from other processes running as the same user but it’d be really interesting to see how far you could get running your entire toolchain using the OS’ sandboxing.