Why can I say "westerns" to refer to "Western movies" but I can't say "actions" to refer to action movie?

Western by some 'convention' turned into countable thing (as per LDOCE dictionary), (one or more films, a collective of films, sort of).
Perhaps they 'transplanted' the grammar rule of plural to "fish/food" that allow plural and singular forms, depending of which sense we use them.

While in the sense of genre it's still uncountable "western movies/western art".
Ref. ldoceonline

The same didn't happened to the words "animation" or "action", it would sound odd to English-speaking natives. By the way, the genre is usually referred to as "animated films" but sometimes they may use "animation" as in the headline "the most succesful animation of all times" (but then, sounds a bit on the informal side).

Audio transcript 1:

"Considering that all three of the given words have the plural form, there would be, a priori, no issue with using them strictly and separately in the plural. In almost all situations, the use of the noun "westerns" is supposed to refer to a certain style of film; therefore, that information would be explicit within the text, making it easy to realize that movie's reference"...

Audio transcript 2:

"When we think of the other ones, actions and animations, by themselves they do not give the reader, at first and without further context, the indication that they're referring to films; once there might be other possibilities, like action sports or action plans. So, that reference would be implicit.
Well...
Concisely, I actually reckon that's only a matter of convenience to make the writing more readable and understandable."

Concisely, I actually reckon that's only a matter of convenience to make the writing more readable and understandable."

In addition to the above, I would like to point out that it may a matter of convenience to make writing more readable, but it also belongs to spoken communication. Looks like the word has been used in that sense in both forms at the time, up to this day.
I can see it for example in an interview from John Wayne to Playboy magazine (it's on the web):

PLAYBOY: Do you think True Grit is the best film you’ve ever made?
WAYNE: No, I don’t. Two classic Westerns were better—Stagecoach and Red River—and a third, The Searchers, which I thought deserved more praise than it got, and The Quiet Man was certainly one of the best...