Because anybody who wants power that badly should be automatically barred from having any power. Obama and Romney have a bad case of Barney Fife-itis.
Yes, we've all heard that old bit of wisdom. Problem is, it's complete nonsense. Running for President of the United States is a really tough, brutal, exhausting, difficult and massively time consuming thing to do. A person has to really want it that bad to have any chance of winning. Anyone who couldn't be bothered with the primary process is unlikely to be a good general election candidate. As with any big goal, people start off with the small stuff and work their way up. Few are good in the first debates of a campaign, but getting better as they go. Few have polished stump speeches at the ready, but become more skilled as they go. I wouldn't trust anyone with the job that wasn't willing to work their rear end off for it. The presidency isn't the kind of thing you can walk away from halfway through because the opposition is too mean.