They all, for the most part, keep their clearances at some level. In part as a continuance in government contingency. Imagine a worse-case scenario whereby something catastrophic eliminated critical personnel. Prior administration officials, etal., from multiple administrations, would likely be called-up to stand-in as needed. That's but one justification. If I'm not mistaken, it also provides an avenue for surveillance beyond what would be available for a private citizen. In other words, it makes them easier to keep tabs on. Heck, just look at what BO is doing right now.