That's why more and more schools are unwilling to provide feedback - they will tell you that it takes up too much time and resources (which is partly true), but also because it puts the institution in a difficult position to give *honest* feedback without it being misconstrued. We are talking about rejecting a person, and not some inanimate object -- and in a selection process that is subjective.
Put it this way. Asking the adcom for feedback on your rejection is akin to asking an ex-girlfriend/ex-wife why she broke up with you - she may know *exactly* why and can even specify specific things about you, but if that were the case you would also know yourself without having to ask her. Or, there may not be any reason she could pinpoint, except that she felt it wasn't working out - and if she told you that, it does you no good. Or, there may be something that she is too afraid to tell you because she fears it is too hurtful to say.
Same with adcoms. If it was something obvious like a crappy GPA, criminal record, etc. then you wouldn't have to ask. Or it could be simply that you didn't get "rejected" - you just failed to get "accepted" because there's way more other people they preferred over you. Or, it could be because they thought you were dull -- or even ridiculed you by quoting your essays to other adcoms (i.e. "get this, this guy said...." hahahahaha). And the "written notes" that an adcom reader will write won't be detailed anyhow because most of the "real reasons" are left unsaid but implied and known amongst the adcoms.
And on top of that, the adcom who may be providing the feedback may not have been the same person who evaluated your case - and will be basing their feedback on what they *interpret* in the notes written down by the reader, which can be vague enough that even the adcom may not know.
So it's not even like asking your ex-girlfriend why she broke up with you, but asking one of her friends why she broke up with you -- so the second-hand info makes it even less useful.