Why can't a chat platform be a product? Chat has only been free when run by volunteers or funded directly or indirectly by ad revenue or bundling with another service. Slack is not MSN messenger, it's an team communication package for companies in the same way that, for example, Zoom is, though the two have different primary focuses.
Zoom always just works. My employer has cycled through a half dozen video chat conferencing tools and had to keep going back to Zoom because they absolutely nail the audio and networking code. Worth every single penny.
Customers are fickle and it wouldn't take much for them to start moving to another chat solution.
Case in point - we use slack at work, but when we need a call we'll typically use discord instead, as slack is horrible for calls (blurry text when screen sharing, often doesn't always let everyone on the call, etc). If everyone in the company started using discord for text chat as well, there would be very little reason to stay on slack and keep paying, as it wouldn't be worth it. Slack don't have something like office 360 to keep companies interested if chat fails to deliver, so all of those customers could evaporate pretty quickly.
IMO it's too easy for competitors to create a similar level of functionality for it to be a standalone product.
If Slack charges too much, there's a host of free alternatives that can be used, meaning they have limited ability to increase prices.
Compare that to office suites. There's not that many options here and it's pretty difficult to build something that would have the functionality that enterprises think they want/need and compete with incumbants like MS.
The main free alternative (libreoffice) has had recent struggles with funding, due to the level of work needed to maintain and improve their offering
