> why don't more companies just set up their own IRC server?
Why don't you grow your own food? Why don't you just write your own payment processor?
I used to think this way until I started a company with employees. My opinion on the matter changed almost instantaneously.
1. One does not simply set up their own "X". Setting up X takes time and you will also have to manage it, which also takes time (and is harder to anticipate when you are fully responsible for managing it).
2. As someone making decisions on this type of stuff (eg a founder, executive, head of IT, etc), maintaining something like X is a poor use of your time and not something you want to have to even think about. So you'll be paying for it anyway even when it's "free" by having an employee maintain it or sacrificing your own time on it.
3. You may trust yourself to be able to maintain it properly, but if you delegate it to an employee, the math changes. Now you have to hire someone you can trust to do a good job maintaining it and who is willing to do that kind of work. And you need to consider things like "what if that person quits/retires/gets sick" and "what if they get hacked or go rogue" and "what do I do when they go on vacation"?
4. Businesses have a lot more requirements of their communications software than just communications. If an employee loses a device or gets hacked you need to easily revoke access to everything, so you probably want to use SSO/an identity provider to make that easy. If you get sued you need to have a way to do legal discovery.
5. If you run into a bug or make a mistake, and introduce downtime or lose chat history, you just lost a lot of productivity or valuable info - operating things like this reliably is expensive, whereas having a support contract really reduces tail risk/provides peace of mind.
So if your option is to pay someone $10k for something like chat that Just Works (99.99% at least), and you are actually trying to build/operate a business and not just play around, something like $10k for chat can be an amazing deal compared to the true costs of doing it yourself. You pay someone some money and barely think/worry about it anymore, done.