I am making a Fischer-esque repertoire, E4 as white and KID and Sicilian against D4 and E4 respectively. But I cannot help but wonder why almost all the world champions except for Fischer had very wide opening repertoires. Is it boredom, to avoid prep, or something else?
I am guessing it was a) To avoid prep and b) To avoid the opponent's strengths and/or your weaknesses.
E.g. if you know your opponent loves the Sicilian and you don't like playing against it, you will not open 1. e4 even if you do that against people who play 1.... e5 as a matter of course.
Also, fashions change. The Italian game didn't used to be so popular (say, 40 years ago) but is now coming back, probably partly because of the rise of the Berlin against the Spanish.
Also, you may be saving your prep for some particular tournament.
Finally, I do think Carlsen is bored and that's why he's playing a lot of really odd openings even against pretty strong opponents.