That is the actual reason this happens. Another one is financial people trying to cut costs or something. Slack licensing is not a small cost if you have lots of people.
However it should be said that IT and finance are supposed to support the company, not the other way around. If they’re choosing shit services they should be replaced with people who actually support the company.
It’s because they did at least one complete rewrite, possibly two. But both of them needed to integrate with the rest of Microsoft, which practically forces them to be crap.