Because it's a perfectly legitimate excuse for drinking a lot. All day long. And pulling off silly shenanigans like kissing people (because they're "Irish") or wearing lots of unmatched green.
And also because you can go around using your new Leprechaun Name....
What's yours? ---> Leprechaun Name Generator
(it comes up with a new one every time, so it's a little unpredictable)
(But more importantly, don't drink & drunk dial!)