Some people have a fair bit of disposable income, and buy more than "that one game I really want".

So they'll play it and it'll be mediocre, or they'll get busy or something.

It's a shame, but that's how it is.

Many people abide by the "pirate first, buy if good" mentality. This can often mean that they finish the pirated game and then buy it on Steam to support the devs. Even though to the outside world it looks like they never played the game at all.
And them fucking sales, man. Psychonauts for two bucks? That's even cheaper than not buying it at all!

>implying I'm not expert at everything

>This can often mean that they finish the pirated game and then buy it on Steam to support the devs
only the most hardcore of fans would do this
most people just buy the game and lose interest
games are active entertainment instead of passive
you have to put the effort in to get to the end, unlike a movie

I've bought my share of games that I've never gotten around to finishing.
sometimes games just suck to much to finish.
Weather it be that the reviews for a game were over exaggerated and avoided giving a game a bad name for whatever reason, or maybe buying a game on a whim you didn't know much about but thought might be interesting all the same and worth a chance.
Sometimes after loosing to a boss 40 times people just say screw it and give up.
some people only play a game for x amount of time each day, and things come up so they can't play that day, then the same the next, then they forget or put it off, and it keeps going until you forgot all about the game.
and by the time you remember and look back, you might think "damn, I don't remember what happened in the first half of the game" and you might not want to replay the game from the start to refresh your memory.
There's a few reason why someone might not
finish the game, but if I had to guess, I'd say most of the time it's just because they thought the game plain out sucked and/or wasn't worth the time.

When I still played games I bought one and didn't stop until I finished it. The only time I didn't finish was rentals.