> I have a theory that desserts that are only on holidays really sort of suck or they would have broken out of the holiday and are just getting by on the nostalgia factor
People have time off on holidays, and families all get together increasing the available labor pool. Accordingly holiday foods can be more labor intensive than foods eaten during the rest of the year, which is an alternative explanation for why some foods only appear at holidays.
As another commenter mentioned, egg nog is one. Home made egg nog is a fair bit of work, doesn't keep for long (well depends on if you pre-mix the booze I guess), and is calorie dense enough that eating it year round would be a mistake.
Heck home made pies in general.
Also certain foods are a lot of work to get setup, but they scale up very well. It is just as much work to mull a little bit of wine as a lot of wine. Holiday cookies, kind of both. If you are rolling out cookie dough mine as well roll out a lot, and if you have lots of kids around, free labor to do the decoration!
As an aside, store bought eggnog is not good, if you don't like eggnog, try home made, you can adjust consistency, flavor profile, and sweetness dramatically. I never liked store eggnog, but on a whim I made it at home once and it turned out grand.