Maybe so. But apparently demand for a product with chat as its one and only one feature is very high.
I think the product is facilitating remote collaboration, chat is just one very significant feature.
There's a lot more messengers than ever before. Signal, Matrix, iMessage, FB Messenger,...
If you were to somehow manage to convince exec or director level leadership to implement something like IRC as your main communication platform, I'm sure they'll be more than happy to let you be the person who's full time job becomes fielding helpdesk requests from non-technical users who are struggling to use IRC :)
Tools like Slack and Teams win because they're all-but-frictionless. One of the competitive edges that Teams has out of the gate is integration with an existing account that is tied to your entire workplace identity if you're in a Microsoft-heavy shop.
It's really easy to assume that things that are easy for us as devs are a "no brainer" for everyone else, but they really aren't. My parents recently had to begin using chat apps for work under COVID restrictions, and although both are plenty proficient at general computing, IRC configuration would be an absolute nightmare for them.
Not to mention the bigger issues of security, log preservation, HR compliance, or any other myriad things that a toy implementation of IRC wouldn't offer. What happens if you lose your backups? What happens if your server becomes compromised? What happens if an employee textually assaults another employee through DMs and you need to audit it? These are all very real questions that people ask, and are questions that Slack and Teams have very sleek answers for. It's not just emojis - it's letting someone else handle the headache of maintaining a critical piece of your communication infrastructure.
Sure you could build that on a small server with open source tools, but is it really worth the effort to build and maintain it for most people?
RIP Slack: It turns out IRC-but-with-emoji isn't really that compelling.