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.

I don't think M$ does much dogfooding. The kinds of issues I encounter being forced to use their pan-awfuly for work makes me very skeptical of this idea.

I am for my day job. I still mourn slack and gsuite.

It is fairly ok for meetings and calendar integration.

It is dogshit at chatting, however.