That's easy.
Discord is made up of closed communities that aren't centralised.
Aside from being underneath a layer of secrecy,needing one to actually have an account and getting the invite link,even if a really massive stick in the mud were to report it,it would only be that one server that gets taken out.
Unless it was a very long running scanlation team that has literally done 1000+ comics,that single smack wouldn't be as big as the hit Mangadex has now.
Discord,despite actively keeping an eye on all the images,has no method at all for stopping comic scanlations,only for detecting actual child pornography,right down to even smacking when a scene from a CP video gets detected despite not being CP in itself.