I've never been bored riding outside. Sometimes I'll get a little bored on my rollers, but not often. The rollers have a certain zero boredom effect because if you don't stay interested you wind up on the floor. It's rather like that outdoors, too. You want to stay alive, you stay in the present on the bike. There are so many things on which to stay concentrated. It's very hard to do everything right, every second. I hear it's called concentration. It's said that champions are also champions of concentration.
However, if one is not really interested in riding as well as is possible, given one's limitations, and one rides where there is no beauty, I can see where it might be boring. I think it takes a certain amount of drive to do one's best at anything and what one is driven to do well at is totally a personal thing.
__________________
Results matter