I think there is definitely enough money to interest MS and Cisco, as we see with MS's move to compete with Slack.
MS, more than any company is very well positioned for this - they have the relationships and sales force.
Cisco has that, but in kind of another way.
Google has the tech, but they need 'Enterprise Oriented Product Management' - which they don't quite have, and a kind of Enterprise Sales, which they don't quite have.
Their Cloud offer has suffered due to this, granted, that's a much bigger nut to crack, with other kinds of overlapping issues.
I think if Bezos, or another CEO decided it should be a focus, it would be.
I don't think Salesforce is hot on this area yet, but it wouldn't surprise me if eventually they did, as Benioff seems to be keen to extend horizontally, and they are good at integrating stuff.
Good on Zoom though, I hope VC's take note and back quality efforts in classical markets.
Combine this with the dominance of Cisco and Microsoft and you have big giants who don't see enough money to complete with other giants.
So the insurgent player is the only one who breaks through.
btw, doesn't Amazon have a similar software (Chime?). So they are competing but only really recently.