Seriously? Regardless of your view on seat belt laws (or drunk driving laws for this matter) the difference is blindingly obvious: A drunk driver is _highly_ likely to kill someone else (I think I may have heard in the past that they are more likely to kill someone else than die themselves, although I'm not super confident on it). If you aren't wearing a seat belt, you're just going to kill yourself.
While this does seem like it's true [0], at least in that case the other passengers know whether or not you are wearing a belt, and can, in extremis, choose not to ride with you (I'm going to ignore minors for the moment as that is a different and much more nuanced topic). When it comes to drunk drivers, other people on the road don't even know they are taking the risk. And of course, that ignores the fact that seatbelt laws do not differentiate between when you are driving alone (extremely common in the US) vs. with passengers.