It's because the other candidates know that a politician is more than a policy analyst, much more. A politician is a representative of the people who elect him. I mean someone who represents an idealized version of what the majority of people in the politician's district want to project to the world. The electorate doesn't just elect people based on their policy positions. In fact the overwhelming majority of people don't vote for politicians based on their policy positions at all.
People vote on the virtues of the candidate.
They vote on image. They vote on credibility. They vote on community standing. Electoral politics is a popularity contest, not a policy debate. Policy debates happen after an election, not before it.