You can ignore them yourself, but people will get annoyed, take to social media, post "this project ignores PRs" "dead project" "they're not inclusive" etc. without context, and those things can gain momentum out of nowhere and have real consequences. And there will goalpost-shifting again, from "just mirror to GH" to "just accept PRs on GH too, why are they being so obstinate" and then further along.

The Linux kernel is unlikely to get those and can survive that even if it does, but your average FOSS project can be severely damaged in terms of reputation, contribution, usage, etc., by this kind of stuff, even if you want to dismiss it as just "social media fluttering". Easier and safer just to keep things clean and clearly at one place.