There are usually two main factors that go into why someone watches streamers play video games.

1) People watch them because the streamers themselves are entertaining. It is no different than watching a vlogger on youtube or an off the cuff podcast. Most of the biggest streamers can play literally any game and still have a huge view count, and it is because they are simply entertaining. Combine an entertaining person with a hobby that many viewers share, and it is easy to see why people get enjoyment out of it.

2)People watch pro players or streamers who are very good at the game they play. People watch them to learn and get better themselves at a particular game. I fall into this category, but of course you eventually start following a subset of these streamers based on their personality, leading back to point #1.

There are, of course, other categories for streamers, but I think the two I listed are the bread and butter of streaming. I personally don't really watch any of the "entertainment" streamers that play a bunch of different games. They usually play single players games and, like you, I would rather play the game myself.

I am a huge fan, however, of watching streamers who are very skilled in their game. These games are almost always online competitive games. You watch to learn from them, to see high level competition from the point of view of the player, etc.

There are also competitions themselves. I imagine I don't need to explain this, as it is like any spectator sport. People watch really good players compete in a game the viewers also play/enjoy.