The Short genre is redundant

This is an interesting suggestion, thanks, however, the reason that we are able to place shorts in their own category is precisely because they have the genre "Short" attached :-)

But when you add a new film title, you have to choose whether it is a feature (over 45 mins) or short (under 45 mins). This is unconnected to the genre.

Consider this too:
Sometimes a title may be incorrectly listed as a "Short" by mistake (or perhaps a developing project's projected running time might change from short to not-so-short).  In such cases, IIUC, an edit request can be submitted to remove the "Short" genre from the title. ... Conversely, if a title is not listed as a "short" but needs to be, then the "Short" genre can be added to it.

Apparently, in the sometimes wacky world of IMDb, the Short "genre" is key to how the database recognizes whether a production is a short ... (even though everyone is aware that a short running time is obviously not really a genre).

Anyhoo ... if that's how it works, well then, that's just the way it works.  (-;
Sometimes that may be the best or only answer you can get!

Upon investigation, it seems like it's possibly the case that selecting "Short" as the length of the film automatically adds the "Short" genre. That's an odd system.