Firstly, I don't believe that "virtually every manager believes they can turn a player around". There's no evidence to support that statement.
Secondly, where are the "major documented fitness issues"? His body-type notwithstanding, he's made plenty of appearances at other clubs.
Lastly, I'm of the opinion that Warnock is neither the type of manager nor in the position to openly take risks of the magnitude of signing someone with "the attitude and fitness of a Sunday pub league player". Particularly for the reported £2.9M fee.