There's not necessarily walls in the package manager model, in most cases it's possible to install binaries outside of the package manager.
It's just less convenient.
It's just less convenient.
And it isn't insecure. If I trust the developer and get the software from them, it's just as good as trusting a repo maintained by random internets who have been known to not only not keep software in the repo up to date, but actually introduce vulnerabilities that weren't there before!
How did you come to this conclusion? Did you request the package be included in the distro you use? If so, and it wasn't provided, provide the bug report/feature request link ...