Slack, Teams & co flat-out ignored calls and screen sharing until now. They were sidelined so-so features. Hell, on Slack you couldn't even see screen sharing from mobile, and you did not even got a notification that another person is sharing. Never mind missing even the most basic annotation features.
No wonder Zoom eats all the pies now. Sure, they made a lot of mistakes, but it was a huge landfall on a small(ish) company. That said, if the others top up their game, I'm happy to get rid of an additional app - but that's how market works.