Because PDF is supposed to be a presentation format, not a format for editable documents. It can be a printout format where the formatting is lost when copying or a bitmap dump of images or it can be text that is editable. It is for viewing not for editing and that is intentional.
Orné'S PDF to PUB conversion can “open’ files from your PUB to PDF Converters. Hathorne PDF to PUB conversion works well, because it's not tied to any particular PUB or MOB format, but rather it can open all available formats. PDF Converters can also be opened as PUB files with no additional costs. I can't recommend them, but you can use them to convert files that you get on your computer or in the mail.