For the same reason that a lot of early HTML wasn't well formatted: the standard is mostly meant for logical formatting (headers, body, etc.) while publishers are more concerned about physical formatting. This leads them to abuse the standards, which works well for some software but not other software.

As per the original question: no, I won't pay more for a well edited ebook. On the other hand, I would probably pay less for a sloppily edited ebook. Within reason, of course. In other words, don't expect me to pay paperback (never mind hardcover!) prices for a lower quality ebook.