1) its fun to see the game from a different perspective sometimes. 2) a lot of people come for the environment and banter that some streamers bring to their audience; as well as their ideas, information and so on. 3) because some people aren't really too skilled at said game, but they enjoy it. For instance, I really like The Witcher 3, but Im complete garbage at it. So, I watch Gothalion's Witcher gameplay because its a fun environment, he makes jokes that cater to my type of humor and is informative about the game. I hope this helps you understand why people watch instead of play, I saw this question frequently and many find it hard to answer.

I'd also add that the "seeing from a different point of view, can open up ideas and influence the way you play the game. Particularly if you're not terribly good at it and looking for inspiration. Even down to simple things like the way many people begin approaching their engagements. Another example is kraftyy, Ive changed a lot about my own playstyle from watching him, how he leads people to where he wants to engage, which leads to 1v3 turn arounds, how he snipes in general, different map routes/locations to keep an eye out for or utilise myself etc. The simplest answer is, there are always new things to learn, so it's easier to learn from people that seemingly play it for a living, or at bare minimum have spent a greater amount of time on it :) Just my thoughts anyway lol