Because it comes 'free' with an Office365 subscription. Embrace (<<you are here), extend, extinguish.

It's usually 'management'. The same management that won't pay for developer tools (including Slack) because 'why do you need that when you can do 95% of your work in VSCode?' It's also usually the same sort of management that can do 95% of their documents in... VSCode and markdown. Or LibreOffice.

Microsoft products are only free if your time has no value.

Having been in the position, on a corporate Active Directory network it very much easier to roll out Teams than anything else. It works fine at the kind of internal video calls that companies spend their days on.