Notwithstanding that it’s wrong, as far as debian rubies go:
Because going back to non-packaged non-vetted flavor-of-the-month code is a retrograde step back to 1993. You lose consistency, you lose the ability to reliably recreate a same environment, you lose tested and low-friction security updates, you lose dependency management, you lose the security of a crypto web-of-trust, and you lose the google-fu of being on the exact same versions of software as thousands of other people.
“If you can’t be with the one you love, love the one you’re with.” – Billy Preston