Dumping everything into /usr/local doesn't even work well when you're the only user on the system: most packages don't define an "uninstall" make target, and installing multiple versions of things is often not well-defined.

See e.g. checkinstall, GNU stow, encap, and probably several more that I'm missing for examples of people trying to fix the crappiness of ad-hoc (non-distribution-based) package management.