Wasn't PDF supposed to be a read-only, print-only medium? You're supposed to edit the source material (like a doc) and export that to PDF, not edit PDF directly.
Well, there's Acrobat. I think that the issue is that PDF is a deceptively complicated format. It's basically a programming language for describing pages to print. So it's not like parsing a JPG or something, you need a PDF interpreter. And I imagine the Adobe one is encumbered.