Well, you need to buy the game, you need the hardware to be able to run the game especially if on PC and it's a demanding game, some games are online only so if they're a console player instead of PC then they need the annual membership also, also if it's an online game then you might be able to do fine with random players but you may also need a serious group if you plan on ranking up in level. Watching gameplay at Grand Champion vs playing at Gold isn't the same thing, and is usually less frustrating. Not to mention that highly ranked players can teach you how to improve and adapt in your own gameplay.

When it comes to single player games, like speed running, I'd rather watch a clip of the one time a speed runner was able to pull off all sorts of tricks with near flawless execution and get the random chances of various things occurring required for a good time than play the same game for 20 hours a day for 3 years straight to maybe accomplish the same thing.

Also, some people are just entertaining whether they're playing a game or just waiting for a game to load. I may not agree with what others find entertaining, but it's subjective so whatever. Others may have injuries maybe in their hands or nervous system preventing them from being able to play well, though it's highly commendable for those that have found workarounds to be able to play (like rigged up controllers for their mouths or to workaround physical deformities preventing them from using a regular controller).

I get why people play and why people watch for both videogames and sports.