Bandwidth/Transport costs for live streaming, especially in group conference call scenarios (where N streams needs to be broadcasted to N-1 participants) become prohibitively expensive after about 8 or so participants unless you can offload those bandwidth requirements to other places (e.g., like in a peer-to-peer architecture).

How Zoom manages to do this in a client-server fashion and is still financially solvent is also a question I've had for a while, but like others say, discounts on the transport and peering arrangements will be a key part in making those economics work, as compression and storage are relatively solved problems here.