Grammar case (wikipedia)

Cases are a thing in languages with inflections such as Latin, Greek, German, slavic ones, and quite a few others. English used to have them in the middle ages, but they faded away in favour of prepositions, and the only place they still exist is who/whom.

Apparently he/him/his and she/her/hers are also inflections, but they seem different enough to not "feel" like inflections of the same root (especially she/her). Since inflections are not a common thing in English, one conceptually doesn't even know they aren't seperate words but deeply connected on a morphological level was opposed to particles (by, for, of, with). Especially someone without the context of knowing how they work in languages that utilize them on a more fundamental level.

However, looking at these words as if they're particles and not inflecions is a simple enough way to know how to use them (that's why you probably haven't heard of them).