I would take a guess that a higher bitrate = longer loading times, and viewers care far more about an extra few second of buffering than they care about audio quality, especially when they don't have the original to compare to.
With newer codecs, doing 4k with 2Mb/s isn't unheard of.
For audio, on the other hand, 32 or 64kbps per channel isn't unheard of.
It does not. That's a recommended bitrate for a live streamer (Streamer -> youtube).
Coming out of youtube, the numbers are quiet different.
For example, a random 4k video I just pulled had a video bitrate of 4.5Mbps.
A 720p video I pulled of a talking head had a 369kbps video stream.
That is to say, a podcast style video is likely to have a nearly 50:50 split on audio/video.
YouTube uses variable bitrate for audio, which can vary dramatically in size. Your example of podcasts or "talking heads" is actually perfect. Most encoders are extremely efficient at compressing voices, as they will only have to encode 30-300Hz, and voices have less data variation than images.
Image encoding is just very complex. It'll get better and better, but audio encoders of the same generation will also improve.
To support seeking you could encode a low bitrate stream, and a high quality stream, and then a number of ramps between these. So when you seek you start with the low bitrate stream and then after a few time units go on the ramp to the high quality stream.
… while yeah… a lower bitrate upfront might lower the required bandwidth and thus, latency, to get enough of a buffer to start playback … all the bloat on the page would be a better first port of call.
How much is loaded for a navigation from one video to the next?