Yes, upstream vendors would love to have total control over how any user uses its software. But there is no guarantee that at least
one of the vendors will not have a really shitty install or uninstall script which will do something like this:
#!/bin/sh
DIR=/home/$USER/ftp
rm -rf $DIR
rm -rf /home/guest
adduser guest
.... assuming that /bin/sh is symlinked to bash (it isn't always), assuming $USER exists (it doesn't always), assuming the user 'guest' doesn't already exist with important files to keep... you really don't want to know how many ISVs actually have install scripts like this.
Users can still install ISV-packaged software in distros. But ISV packages typically hurt the system by removing things like vital dependency tracking information, or replace files that other packages provide, or won't be tracked for security patches, or will be installed into non-standard paths.
--
I have been a package maintainer for 6.. no wait 8... no, 9 Linux distributions. I have created thousands of packages (easily 10,000). I have maintained forked distributions for corporate use. I have also developed dozens of individual software projects which all were designed to be packaged by maintainers, and were adopted into Linux distributions by somebody else.
And i'm telling you: Anything other than maintainers packaging software is a fucking nightmare for everyone involved. We have a good system! Devs, make your software so it's easy to package. Maintainers, package it for your particular distro. Users, just use what's in the distro, OR follow the upstream's instructions on installing _at your peril_.