I'm wondering why are cranial nerves not included in the central nervous system?

Mostly peripheral: Cranial nerves have their cell bodies within the central nervous system (cns), but the majority of their length/"processes" are outside the confines of the cns. The myelin covering nerves inside, vs. Outside, the CNS is different, and cranial nerves are subject to disorders common to peripheral nerves. (the cell bodies can be affected by CNS disorders as well.).