Series of movies fall under 'movies' eg MCU, The Godfather, Frozen, etc.
Movies that are released as 'sequels to series instead of adaptations from scratch' I guess fall under 'movies' in this post. But maybe we can just ignore these movies.
There are lot of Western live movies, Western live series, Western animated movies and Western animated series. I don't see any particular bias to certain categories.
When it comes to anime, I notice there are lot of anime series but not really a lot of anime movies.
I checked out MAL just now
They're mostly series. Many of the 'movies' on the list are actually 'sequels to series instead of adaptations from scratch', except notably Kimi no Na wa.
To clarify, there are 2 cases I guess here, but maybe it's the same answer for both:
Case 1 - There are some anime movies that are anime originals. But it seems (ok actually I'm not sure for anime originals) anime originals are usually anime series instead of an anime movie. Why is this the case?
Case 2 - There are some manga (or novels / light novels / visual novels / drama CDs / etc) adapted to anime movies (or series of anime movies) instead of anime series (which may or may not have movies that are 'sequels to series instead of adaptations from scratch'). But it seems the adaptation is usually going to be an anime series instead of an anime movie/s. Why is this the case?
For either case: Maybe there's a kinda compromise in that many anime series are pretty short (obvious exceptions are Naruto, Bleach, One Piece, Yu-Gi-Oh!, Pokemon, etc) compared to Western series which sometimes go on for more than 5 seasons even if they still suck eg Once Upon a Time. Like many of these 'series' are actually better classified specifically as 'miniseries' or something. In this way, maybe there are about the same amount of anime series and anime movies if miniseries were counted as movies.
Eg We might consider Death Note, Hellsing, Lucky Star, Erased / Boku dake ga Inai Machi, The Quintessential Quintuplets, School-Live!, NTR: Netsuzou Trap, Cowboy Bebop and Future Diary as miniseries.
Ah I found this indirectly thanks to Dimitri mx ♦: Why is it so rare to see new long running anime series these days?
For case 2: Maybe manga is usually made with expectation of anime SERIES, not MOVIE/S adaptations? As for the few cases of manga that becomes anime movie/s (WITHOUT or at least before there's an anime series adaptation), I guess it might be the same reason as why some manga are adapted to anime series initially and then later there may be movie adaptations that are 'sequels to series instead of adaptations from scratch'.