Why isn't cam.* part of the uk.* hierarchy?

Because it's not. cam.misc was created as a local group which is carried by multiple sites back in the days when UUCP was the dominant transport protocol and sending mail meant you had to know the bang path. As it happens most of the IP providers in the UK carry it but it is not part of Usenet. The other cam.* groups followed and were created on the same basis.

It has been suggested at various times by various people that a uk.local.cambridge and/or uk.local.cambridgeshire hierarchy should be created, but nothing has happened yet (there's more on this here). The most relevant uk.local.* group at present is uk.local.east-anglia