It's not hate so much as a feeling that it's overrated. Whenever anyone describes this game to anyone else, the Milkman Conspiracy is inevitably cited as an example of the game's brilliance (and frequently the only example, to the detriment of its other brilliant levels).Again with the Milkman Conspiracy. Why so much hate?
I don't really agree with Craft's critique of it, though. TMC is highly linear, so I'm not sure how one could get lost so easily, and the reason it uses the same theme/joke over and over is that it's basically an exploration of what happens when you take the "mindscape" concept of Psychonauts and apply it to the mind of a paranoid schizophrenic (combined with multiple personalities, as so many works are wont to do). So naturally, a lot of the level's artistic design is based around the idea of seeing everyone and everything as working against you, seeing conspiracies in every shadow, etc.