Because it's fun and it's a way of celebrating Mexican-American culture, which we need to celebrate more than ever imo. Some people need to live a little.
The only think I would ask is, if you're gonna celebrate it, at least learnt the history or what are you actually celebrating. Cinco de Mayo ain't our Independence Day guys, it really isn't.
I have no problem with people drinking, eating, being jolly on May 5th. But I do have a problem with people wearing stereotypical Mexican outfits while yelling things like "arrrrrriba!"
just cut that out, dress like you always do and get drunk