On the other hand it is also not clear what is the audience of people who don't play games but still watch esports (like me).
It's not like the case of football, where you need friends and you need to go out to play. Thanks to matchmaking systems in games, you don't even need friends to play. You just click a button.
In fact this is part of the problem. the Dota-like/MOBA genre indeed has decent match making for the major games, but having a team you know, are communicative with and have a long/deep experience with one-another's playstyles helps immensely. I only really played League, so I can speak mostly towards that, but it's a different game queuing with friends than it is queuing with randoms. A lot of the excess communication required for a smooth match is removed when you have established a "sense" of how your teammates will react to situations, and it changes your judgement a lot when you no longer have as many random variables to consider during team fights, ganks, etc.
Watching pros play does a few things as well; you see not just good strategies, but mathematically sound and consistent strategies, which make the times you do play much more fun since a lot of your guesswork is removed. Pros also tend to have the skill for higher performance, and you get a chance to see pretty unique situations and plays that just never happen in ranked/normal queues. Plus, the pros have a huge audience typically, and you respond with the audience; being excited about an awesome play is amplified when you and thousands of other fans are collectively "losing your shit" over a play.
So yeah, you can go solo in such games, but the experience is always better with a team of people you know and can trust, and you end up in the same situation as football where you need to find others.
Sorry for being snarky. The point is people are going to be drawn to those that are experts in just about anything they care about or are interested in.
I don't like it myself. But a number of my coworkers, and cousins, are of this cohort.
In general, it seems like they want an internet personality to follow. They like watching someone who gets angry and frustrated as they lose, or gets excited when they win. Its no different than following the personal lives of celebrities, except these "Twitch Celebrities" or "Youtube Celebrities" are more personalized and tailored for your specific interests.