For similar reasons that no fully compatible MS Word clone exists: very high complexity of the format, underspecified format details, and undocumented implementation algorithms.
Even if the format is understood, it's also very much a case of Acrobat handling extremely badly formed PDFs instead of erroring out if it were strictly adhering. If you've ever seen a save prompt when making no changes, that happened.
"It works on Acrobat" was something we got told a lot from clients sending us PDFs that were broken according to the official spec - we just had to deal with it.
