Many s/w projects still maintain support channels on IRC.
The migration to proprietary support mechanisms isolates a number of users. Mozilla in particular is much more difficult to interact with since migrating away from IRC.
I've also worked for a company that ran it's own XMPP chat server. XMPP chat clients allow file attachments, see messages from times when client is disconnected (without a bouncer), integration of voice and video chat, whiteboard, etc. Pretty much everything slack, discord, etc offer.
This question is similar to why do companies use the cloud, when most of the time they could just run a server. My biased opinion: laziness. Mostly the laziness of following fashion instead of the technically superior solution.