>>36834528main game = ez baby mode that doesn't do enough interesting things
the experience of capturing and raising monsters while traversing a region is nice (regardless of difficulty), but only for so long. it highly depends on how acquainted you are with the games already, and how good the region & everything in it is. if you've played enough of the games for the basic single-player, pre-champion experience to be old news, and the region doesn't offer much to spice that experience up or warrant replayability, then of course it's going to be boring. after nuzlocke runs or other self-imposed challenges, the post-game content and multiplayer stuff are the only remaining providers of continued entertainment, and so are the last straw for people who've already played a fair amount of the mainline games.
if the meta of whatever online formats one plays are stale or otherwise not fun (or they're not interested in multiplayer at all), and the post-game stuff is minimal, then there's no reason for an established fan to continue playing a title beyond challenge runs (which they've probably already done enough of too) or just for a nostalgia trip if it's an older title. if someone is satisfied merely after the main game and doesn't care for post-game stuff (these things also coming from a product that they paid money for and may have waited for the release of), they either have low expectations, may not care enough to really be "interested," or have a smooth brain.