Perhaps it's just a value comparison thing. My company pays $5/user/mo. to GitHub for private repos, storage, and GitHub Actions. Every developer uses it every day to accomplish business critical project work. A pairing tool feels more optional to me, especially when Slack's built in screen sharing accomplishes 50% of what we need as part of another service we pay another per user per month fee for already. So a tool that's 5x the cost of GitHub that serves as an optimization for what we do is hard to justify. As a company we could afford it, it would just be a difficult sell, especially when a competitor is still free (turns out "for the duration of the pandemic" is a lot longer than they thought, oops). Of course, if we had a culture that paired on a regular basis, this might be easier, but it's more of a when you need help or want to work together thing, rather than being required.
Additionally, as others have mentioned, I think some of the issue is the lack of a free or low cost personal tier that has some set of restrictions. What those would be for an arbitrary pairing tool I don't know, but it feels really pricey if I just want it available for one-off pairings.
Now, personally, I could sign my OSS project up for a free version, but since I don't need (or want, lol) to pair with most contributors, that feels a bit wrong.