The reason that the average payout is so high can be linked to a few things: one is that the cash spaces have gone up in value. If you get money, it is a minimum of $300. (I don't know what the average is.) Second is the prize puzzle every show that has to go to someone. Third is the longer puzzles that add consonants to what would be a shorter round (Proper Name GAME SHOW HOST ALEX TREBEK instead of Person: ALEX TREBEK, for example). Next comes the $6,000 in toss-up money that has to go somewhere.
Perhaps the biggest is the speed-up round. In the beginning, the speed-up round was played as a fast way to end the last round of the game. These days, not only is there always a speed-up round, but consonants have a minimum value of $1,300. That's money that will find a happy home too.
All of that adds up in a hurry.