Cause they need an excuse of 'a leak' when the politicians 'f' up and make the wrong call. It's called hedging your bets. Sometimes, counterintelligence can be used in such a manner. Of course, the public will never know the seriousness of any security breech if those stolen and lost laptops had real sensitive data. IMHO, no government employee should have any type of sensitive data stored on a mobile laptop. But then again, I reckon government and sense will never mix unless shaken very hard.
In today's world, I can understand the "need" for laptops in the field. But they can be made secure so that the data is safe. Encryption, passwords, keylocks, etc.