The rationale is to minimize the number of clearances. Even though you can't get information without a need-to-know, the government still likes to limit the number of clearances.
Interesting, I didn't realize it was considered an ongoing process. I guess that makes sense; otherwise a bad actor would just apply for clearance, get cleared, and then start doing things they shouldn't.
They don't actually tell you explicitly it is an ongoing process, but everyone with clearance I know seems to acknowledge they are probably being kept tabs on at least a little. I mean it would seem kind of irrational to have all these watch lists and not watch the people with privileged information.