The PS3/4 games aren't particularly challenging either -- the difficulty, if any, mostly stems from wrestling with the floaty controls. They don't have infinite lives, but you get a set number of lives per checkpoint, and there are enough checkpoints that you'll rarely, if ever, fail a level. There's also multi-player sections of levels, and special challenge levels.
Also, there's the level building features of the games that many people probably spend the majority of their time. Oh, and user-created levels can be more challenging, of course.
I like the games for their whimsy, the art style, and their heart-warming presentation.