Why do people watch sports?

Because they can't or don't play any sports themselves, so they become players vicariously.

Because they find watching sports entertaining, just like you find watching shows and movies entertaining.

Same reason we have wars , Its called tribalism & its the instinct to Identify yourself as a member of a particular group & dominate another group.
Spectator sports are simply socially acceptable ritualized war without much death or bloodshed.
Its a basic primate instinct in all humans & its usually first manifested on an individual level at around age 3 with primitive games like keep away with toys & king of the hill over particular spaces.

Entertainment

They want to see their team win.

I didn't start watching sports until I was like 15

Then I realized hey, these are the best guys going up against each other...I like watching people compete also...so that's why I started watching sports

Conditioned to entertainment

Why do people watch anyone do anything that they could be doing themselves? Because it's fun dude. I can't speak for sports because I don't watch them, but the same way you'd rather watch someone run around chasing a ball in the rain instead of doing to yourself, I'd rather watch someone play a horror game so I can be less immersed and hopefully keep my pants dry. Same principle.

Entertainment

Because they want to betray the American Flag.