I suppose because they enjoy it or think it's FUN!
Spare time AKA as "leisure time" is for doing what you want.
The rest of our time is filled with work or other obligations.
For those who like to play games it's their way of having fun while others enjoy watching and cheering at sporting events where others are playing games.
No one does anything in their spare time that they don't want to do usually. It's their "free" or "me" time.