It's often simply a matter of protecting the company's secrets. The longer you stay, the more you might learn and potentially use against the company if later

It's often simply a matter of protecting the company's secrets. The longer you stay, the more you might learn and potentially use against the company if later working for a competitor. A policy like that does not at all have to be rooted in paranoia. Obviously, most companies don't have any secrets of such magnitude that it would matter, although many like to pretend that way.