Zoom doesn't want to use the stock H.264 encoder as provided by the browser for WebRTC communication. Instead, they use their own video encoders and decoders (which while still being H.264, it is presumedly better optimized for their use case). WebRTC forces you to use either the H.264 or the VP8 encoder/decoder that the browser provides.
How they do this is by having their own custom application that you have to install. Still, some users have noticed that there is a well hidden web-based version of Zoom, which works by again running their custom encoders, thanks to WebAssembly. Also it seems that their video is transmitted via DataCahnnels [0].
They are not alone. Companies want to provide additional "value" by innovating outside of what the WebRTC standard offers. That's nice and all, although it of course tends to disgregation and incompatibilities in the long run. For this reason, I've heard talks about how future revisions of the standard might explore adding WebAssembly support, in order to allow everyone embedding their own compiled components into their applications [1].
[0]: https://webrtchacks.com/zoom-avoids-using-webrtc/
[1]: https://webrtcbydralex.com/index.php/2019/11/13/webrtc-stand...