Is this in reaction to something? I've seen some Zoom security hype, but nothing serious enough that it's going to hurt them. What do you think will happen to them?
There are definitely alternatives to them, none as simple and cheap. Which answers the question of "why not" - there's no profit in it. My prediction is that someone with a very sophisticated advertising and personal-data business model will have a product out soon that competes (or they'll just buy Zoom). Facebook and Google would be front-runners for it.
In fact, apparently Facebook has launched group video chat in the last few days. Haven't looked at it yet.
Google, Facebook, even Apple with Facetime, weren't able to compete with Zoom for large scale meetings. You can stream privately on Youtube for large scale broadcasting (one with the link), twitch (but it's public). Google Meet is tied to their GSuite which is unusable outside their eco-system. Teams works, but it's not as seamless as Zoom. Honestly, I don't know why Skype has taken over for enterprise.