I would like to know why there's no scan to diagnosis abnormalities in generalized epilepsy?

Brain is complicated: The brain is a very complicated organ. Epilepsy is when a person has repeated seizures over time. There can be many different reasons why someone has seizures. Seizures are due to abnormal electrical activity in the brain. Therefore an electroencephalogram (EEG) that detects and records electrical brain activity is use to diagnose seizures/epilepsy.

No structural lesion: There is a lowering of the seizure threshold due to an abnormality of ion channels or receptors. Many seizures that manifest clinically as a generalized tonic clonic seizure have a focal onset that is so brief or subtle that it escapes clinical detection. The EEG may show the real problem. (Onset in a location with rapid spread of abnormal electrical activity to the rest of the brain.).

Seizure workup: Depending on the history and physical and laboratory exam by a Neurologist, EGGs and perhaps brain scanning can be done looking for tumors, A-V malformations, etc. It is up to his judgement not routine.