Viewers go in and out (they dont' start from the beginning)

That may easily be true for TV shows as well.

They may be viewing multiple streams simultaneously (imagine the cacophony!)

If the streamers are just speaking (i.e. no music), especially if more than one of them tends to talk a lot, you get the cacophony as well. Watching multiple streams at once like this only works if the streamers you watch tend do be silent most of the time or you mute some of the streams. In the former case, I agree, it's better not to have music, however, I don't think it's that common for people to watch this way. In the latter case - well, it doesn't matter whether they play music or not if you mute them anyway.

They may be viewing while doing their own work (and only paying attention when something interesting comes up, which is easier to detect when only speech is heard)

I actually do that myself a lot. I can see how it may be a problem if you don't like the music that a streamer plays, but then again, no one forces you to watch this particular streamer while doing your work.

This actually is a part of a bigger point. Many things in streaming practice are based on preference. At first the decisions are made based on nothing more than what the streamer prefers - there is no audience to think about. This means the stream will attract people with preferences similar to the streamer's. After a while making a big change won't make sense any more, because most of your audience will prefer the things the way they are.

It is of course possible, that you could have built a bigger audience if you made a different call in the first place. However, once you make the initial decision, making a switch may be less advantageous than keeping things the way they are.

I don't find it awkward when office mates are silently working next to me, until they have something to say.

Having someone else working next to you is definitely a different experience from watching most coding streams. I treat streams (both coding and gaming), TV shows and music as background noise and something to focus on if I need to take a short mental break from what I'm doing. Having someone else sit next to me and working on their own stuff doesn't give me that. It's nice on its own, but it's an entirely different thing.

Of course, like I said a lot of times already, it all comes down to preference. I'm not saying that your way of thinking about streams is invalid and I'm not trying to change your mind. Unfortunately for you, it seems to me that people tend to prefer the full package, with background music, interacting with viewers and everything, rather than just a broadcast of someone typing code or play games. So streamers will tend to do exactly that - provide a full package.