It can take an enormous amount of compute power to encode 4k video at 1:1 speed with these more complex codecs. Multiply for multiple bitrates and resolutions. If you can't encode at 1:1, you add significant delays to 'live' streaming, thus making it not 'live'.

Case in point, a 40 second 1080p clip would take over 8 hours to encode on an i7-4800 - that's less than 2 frames per minute. You need a lot of horsepower to cut that down to 40s / 60 FPS.

[1] https://bitmovin.com/bitmovin-supports-av1-encoding-vod-live...

Like the other comment said: the point is live streaming, so the encoding has to be done in real-time. Being the slower of the two steps (assuming comparable hardware between sender and recipient), it becomes the performance bottleneck when measured as frames per second.

When measured in, say, total sum of all CPU cycles and hence total energy spent, then yes: the decoder is the bigger deal. So your argument holds for non-live videos.