Because you shouldn't want your coworkers to know you have a drinking problem.
Because chances are, you're not going to order just one, and that could get messy.
Because "bombed" pretty much describes how you'll act at the party, how well that conversation you can't remember having with your boss went, and how your liver is going to feel when you wake up the next morning.
There's nothing wrong with taking a round or two of shots (like say, Picklebacks) because it's a tradition among you and your coworkers. But pounding shots of tequila or Jger like you're 18 again and have a class to skip the next morning is not going to win you any Employee of the Year votes.
P.S. Those drinks generally taste awful anyways.
If it's not an open-bar type of situation and you know the big guy is going to be footing the bill on his credit card at the end of the night, don't go for the biggest ticket item on the menu (or several rounds of a mid-level item). Ordering three fingers of that rare scotch or four glasses of that nice barolo, because you think think you and your boss are better bros than you actually are, runs the risk of being awkward for everyone involved.