Why do Darknesses need to be managed / cleared regularly?

Ch79 in the LN rn.

I know that Daydream Inc wants the dream essence, but what is the canon reason they have to clear a darkness at all every x time period to manage the darkness? Also, why can't they just farm F rank darknesses repeatedly?

Or the Disaster Management Bureau, why can't they just leave the darknesses alone by restricting the entry points? Like not spreading the cassette tapes for that related darkness.