Why do people leave early?

I just don't understand it.
 Why do people leave before a concert or a game is over? Why would they deliberately miss something they paid good money to see?
OK, if the score is 50 to 10, yes, I can see it.
But at the Rider game on Friday, my brother said people were leaving after Montreal pulled ahead with 1:47 to go. Did they forget that anything can happen in the CFL, and often does? The people who left missed the most thrilling moment in sports -- a Riders come-from-behind victory in the last 73 seconds of the game.
And at the McCartney concert last week, the one that people paid $300 or more a ticket to attend, we saw people shuffling up the stairs and out of the stadium even before the first encore. They missed, maybe not the best part of the concert, but certainly one of the better parts -- Daytripper, Get Back, Yesterday, Helter Skelter, the Regina pipe band, and the tattoo girl.
Can anybody explain why people do this?
Remember this one -- I wonder how many left at the middle of the ninth?