If a security clearance is given to an employee (at an outrageous cost) then it seems foolish to remove it when they transfer to another area.
If it is good for a number of years, then allow it to expire or it can be used in another position that is obtained shortly there after.
Mr. A gets TOPSECRET---2 years later he leaves HQ office in CA and goes to field office in TX. He no longer needs TS clearance. It is removed. One year goes by and he is placed into an Intelligence related job requiring a TS. Mr. A could have retained his TS and the government not paid for two clearances within four years.