Companies don't trust sales reps for the most part. They use them, but they don't trust them. Hence the trade secrets clauses in their employment agreements. Companies like to hire experienced sales reps because of their contact list? Sure, because if that employee's former employer hits them with a lawsuit, they can fire the employee and settle out of court. It's worth the potential risk. At the same time, there aren't many companies that are going to stand behind a brand new employee when they know he's probably holding onto data that is considered a trade secret.

What happens when your previous employer fired you for using data you stole from a previous client?

Good luck taking years to build back your reputation.