Clearance requests must be accompanied by a justification, which basically means "needed to work on Project XYZ". You cannot get a clearance unless you actually need it to do work, and should not be cleared any higher than is necessary[1]. This leads to the chicken-and-egg situation with cleared positions: you need a clearance to work on these programs, but you can't get a clearance for the first time until asked to work on one of these programs.
[1] Some companies will try to put you through for higher clearances than you actually need right now in anticipation of future work, but they aren't supposed to.
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.
[1] Some companies will try to put you through for higher clearances than you actually need right now in anticipation of future work, but they aren't supposed to.