There are several ways it can be fun, and you do have one of them.

• Competitive games/eSports. These are fun to watch exactly like football or basketball is, the skill it takes to play the game.
• Speed or challenge runs. Kind of related; imagine watching a skilled mountain climber climb a mountain looking to set a world speed record or something like that. Challenge runs aka “can we do this” can be fun as well. People are drawn to that kind of story, and video games are often a safe yet compelling place to contrive that kind of scenario.
• Venue for an entertainer. Sometimes the audience isn’t really there for the game, they’re there for the player. The game is a backdrop for an impromptu comedy performance.
• To experience the game when unable. Catching a Let’s Play of especially an old game that’s out of print can be a way to experience that game if you don’t have the ability to set up an old console, or if an online game has shut down, or if you just don’t have the time. You might watch it while cooking or eating or doing some other task if you can’t find time in your life to actually play.

I’m kind of the opposite. I don’t quite see the appeal of people watching physical sports if you’re not partaking.

I watch a certain gay furry play games I will NEVER play myself but want to enjoy the story/gameplay of without having to do it myself, like Dark Souls 2 or Celeste.

Polar opposites exist. Letting a brain melt into stew watching whatever reptitive thing that’s happening happen can be relaxing for the holder of said brain. It’s like a pureé of thoughtless goopy wonder. Truly an experience to behold…

Also, competitive gaming is like the nerd equivalent of sportsball. Not bad or anything, but a lot of peeps tend to assume “I like exercise that’s fun,” means “lets have a battle-to-the-death-style sports game.” The same concept bleeds into gaming.

I sometimes like watching videos like this as a way to relax.