Apple cider is seasonal because it relies on apple harvesting, but there is no reason to not have pumpkin spice any time of year except for artificial scarcity. The same with eggnog except people don't like eggnog much to begin with.
But apples are available year round and cider is a great storage method. It is available year round. Preferences and market drivers cause seasonal supply increases.
I've found Costco to have decent eggnog, but the regular grocery stores have utter crap. I've made it myself, but imo it's not worth the effort and storage space.
Actual eggnog is an affront in the fact of god, but the carton of custard flavored milkshake that Southern Comfort sells every year is pretty excellent. I'm led to believe you can even put booze in it.
Asterisk: The beverage you're talking about (cider) is effectively apple wine and can be stored and maintained in pretty much the exact way any other wine can.
When you hear most modern Americans say "apple cider" they mean unfiltered, unclarified fresh apple juice, which is sold fresh in the mid-fall, kept refrigerated because it isn't shelf stable, and often served hot and spiced.
You can thank the temperance movement for the confusion.
Ok, now same question but for the Christmas lebkuchen and cinnamon stars and all the stuff I would stuff myself with all year round if it was available.