Expensive isn't really the issue. For a (reasonably sized) company (earning decent revenue) plenty of things aren't expensive in isolation. The problem is that there are a lot of things a company either needs or wants to spend money on, so at some point you have to start prioritising.
For us, we need single sign-on, so Slack's £9.75/user/mo. For 200 users that's nearly £2000/month, or £24000/year.
We're now nearly 300 people. That's about £35000/year. And it only gets worse as the company grows.
That's more - I mean, way more - than it costs us to license SQL Server (amortised annually on a 3 year upgrade cycle). It's more than the licenses for several of our other business critical systems and services combined.
In terms of priorities then, we're about to embark on a series of infrastructure improvements and I'd much rather use that £35,000 there, where for us it can add more value.
The problem with Slack is it sounds cheap but the cost creeps up to quite substantial levels for even relatively small organisations. It then needs to be weighed against other costs, and the value it provides versus the value they provide.
Whereas on the other hand we're already paying for Office 365, which everybody in the company uses heavily every working day of the year, and Teams is bundled with it, and it does everything we need, so why not use it?