He made 5 title defenses. Two were against the previous generation of players, and he did extremely well. 2 were against players of his generation and were anything but compelling victories. He only won a total of 1 classical game in the 24 played, and that was in a must-win scenario because he had just lost for a final record in these matches of +1 -1 =22. And finally there was his match against Nepo which was looking to be another extremely close match until Nepo lost a critical game, and then went on monkey tilt, as is his reputation - proceeding to play horribly for the rest of the match and get wiped.

In an interview with Rogan, Carlsen stated he felt he peaked a bit before his match against Nepo, and so he probably did not view his chances of success in a world championship match as especially high. So he was going to have to spend months preparing for a match he could very well lose which would certainly tarnish his reputation as the GOAT of chess. I think this is why he couldn't find the motivation.

For instance there were new world records just around the corner. The most successful world title defenses is 6 and that was back in the early 20th century. With one more he could have surpassed Kasparov and at least tied the record.