Last night I was pulling out the 12 to 18 month hand-me-downs from the closet so that they're actually staged as she grows into them, and I figured it out.
One year is just about when you run out of stuff from your baby shower. You need to either throw a birthday party, or start paying for things yourself. Suddenly it all makes sense!
Of course, a couple of women I know threw parties whose cost exceeded the gross national product of Guam, but that's an entirely separate issue.