Because the concept it apes/derives from is so wholesome -
http://gbatemp.net/threads/tomomi-i...i-minegishi-must-stay-celibate-thread.342239/
And because the games themselves are such shining masterpieces of game design (my knowledge here is admittedly a bit lacking but I do read up on every 360 game that gets released, kept abreast of the handheld world and do OK for most other console ones).
There are cases where dress up and fanservice type stuff can work, indeed some of the card and puzzle games with it as a core mechanic are actually half decent implementations of the games they base themselves on* and have mechanics of note as well, if we are going rhythm games with something like it then space channel 5 and I would remiss not to mention that dress up is a fairly large component of probably my favourite game of recent times (resonance of fate). However idol games are to those what pay to win flash games are to networked games in general: far from being deemed unworthy of existing but hardly a cause for high praise.
*the better ones do include an option to say say "stow it and give me the game" mind you.