Because TF2 is better under the influence.

Drunk Fortress 2: A group for people who enjoy playing Team Fortress 2 while drunk, intoxicated, or otherwise under the influence. Non-drunkards welcome too!

While we don't condone overly-excessive amounts of alcohol, we sure can't stop you from drinking as much as you want. Remember we're here to have a good time playing friendly games of drunk TF2! Please keep yourself under control, and don't get abusive or violent towards others.

TF2 Official Blog (Valve)
TF2 Wiki (Unofficial)[www.tf2wiki.net]