Erasure might be a too strong word for it but fandom definitely focuses on men and their stories over women. I'm not saying that slash is the only way that fandom does it or that all slash does this (in my original comment it was specifically about the slashing of m/m exclusively).

And yeah, part of it is that the show/book/etc. in the first place will have male man character/cast made up of mostly men but even in that case you'll see fans taking up minor male characters and rounding them out rather than a minor female character. Canons with significant female characters do tend to be much better about including women in fanworks but idk it seems to be a bit of a cycle? Like there's a small female presence in show -> the fandom focuses on the male characters -> people assume that fans don't want to see/enjoy female characters.