Because none of the examples you listed have end-to-end encryption with perfect forward secrecy for one, including IRC. Also, IRC specifically is very confusing to even make a username on and keep it, let alone know how to do anything else on.
We have never used Discord or Slack, specifically because they don’t have e2ee, but at least they have, from what we understand, a pretty simple sign up process and good UI/UX.
Yes, this is why we use matrix over IRC due to e2ee. PGP last we checked doesn’t have PFS, sadly.
We remember when we used IRC having to remember all those silly commands for nick bots wasn’t exactly good UX.
We said that we don’t use Discord/Slack for that reason, so yes, we are disgusted by them too.