[nataz]

So, This is way more then a background "check", it's an investigation. The government will collect information that you provide, and then it will attempt to validate it with multiple independent sources that they seek out. When you give them names to interview, one of the first things they ask those people are for another list of people to talk to. That's why it takes so long.

Once they issue a clearance it's good for around five years before you are re-up for another investigation.

Because of that investment of time/money/resources, generally speaking once you are issued a clearance, even if you leave government, the sponsoring agency can continue to hold it as inactive for a period of time (it varies from months to years). It's much simpler and faster to activate a clearer then to revive a new one.