Leroux: In general, I'm not too surprised that there are only few games that are explicitly holiday-themed, and that it's easier to find DLCs, mods, events. Tendentially you'd want to sell games that players would feel like playing all the time, independently of the date. I don't know that many Christmas-themed games either, and Christmas is even more widespread than Halloween (in my youth Halloween wasn't even a thing in Continental Europe, for example, it was imported from the US by individual geeks and corporations who saw an opportunity to monetize it).
That's a good point, but most people don't play games all of the time for an extended period. They usually move onto another game, and if they liked the previous game, may come back to it again in the future and play it again. A holiday themed game may be limited because of it's particular holiday theme, but people will often play those games every year at that particular holiday.
Maybe it's a poor analogy, but look at what happens every Christmas with Christmas themed movies. I don't think there are too many movies that get watched more than popular Christmas movies that are shown and watched over and over again throughout the Christmas season EVERY year even though they are decades old.
One more thing. Games like Batman: Arkham Knight are good to play during Halloween but are not directly about Halloween so they aren't limited in any way as far as people only wanting to play them at a certain time of year. I would be happy with more games like that.