There's also the simple entertainment of the game. The game can be thought of as a story that has a beginning, middle and an end that can be happy, exciting or just plain boring. It can also be tragic if things don't go according to the fan's expectations.
I think all of this carries over to why people like to watch streaming games.
Another reason is just entertainment value. The player is funny and interacts with viewers, or the game genre itself is entertaining to watch. For example horror games, it's fun to watch streamers get jump scared.
Yet another reason is to preview a game before buying it. I watch dozens and dozens of streamers, and at this point I think I watch more games (by title) than I play.
For me, it's like watching a game of chess, or a match of basket-ball. I know the games myself and I know what it takes to play it at a high level (I watch mostly professional players). It's an act of learning sometimes, when your own strategies differ quite a bit from the player's and that can lead to some interesting reflections. Also, they are humans, and you can empathize with their joy when winning a game, or frustration when suffering from a bug or bad teammates.
Sometimes it's a spectacle, too, when the streamer puts on a show with an over the top personality. I don't really watch the kind of streamers that do "stupid" behavior, but you must be aware that they represent a sizable part of the gaming population and for them, it's normal to rage and stuff (not my kind of things).
Also, I can't play games for long (not so good internet + work + other hobbies) and when I'm doing other stuff, I will put a gameplay in the background as all the other video contents I consume require focus.
Occasionally I'll tune in for a big game and watch it the way people do sports, especially if my favorite team happens to be playing.
More often though, it just feels like a hang out the same way a podcast or talk radio can. Casual conversation with something in the game occasionally drawing attention.
I should say maybe I'm an edge case too. I probably only watch a couple hours a month.
Edit: ok I thought about it some more and I had one more thought.
Because there is a lot of it.
There are a few people I really like watching in interviews or give talks. But you'll find with any of that kind of content that you've watched it all after a few dozen hours max. With steamers there are hundreds to thousands of hours of content available for the person you find interesting to listen to.
AoE2 and AoE4 both have thriving streaming communities with charismatic, highly skilled (et top 10 etc players) streamers. It also gives you a chance to interact with your game's heroes, sometimes play team games with them, ask them questions about the game, why they just did X etc.