All of those are absolutely true, and many of them reduce to one thing

    - didn't have a frickin' clue what the code was supposed to do

I remember reading the old cartoon (and it was already old when I saw it 25 years ago) of the manager saying "You guys start writing code, I'll go find out what they want", and swearing I would never be that kind of manager only to find, all those years later, that many of my programmers had already internalized the notion and routinely sat down to write code without having a clear idea of what the block of code was supposed to accomplish.