Re: To be clear ...
>I presume it's all about the slurp.
No I suspect Zoom does it that way because that was the way they did it in Webex (remember Webex and Zoom are like WhatsApps and Signal).
I suspect it was done this way so as to keep the client small and have a single stream from the client to the streaming server, thus able to execute on a wide range of systems. Also, architecturally it makes sense - Webex is effectively just an enhanced streaming server - remember webex was designed before today's obsession with communications security. So having the streaming server save a copy of the stream in massive purpose built storage array not only makes technical sense, but also commercial sense as you can make this a chargeable feature...
Also remember Skype was originally a one-to-one telephone call replacement, not a one-to-1000's conferencing solution.