They're thinking that if you had something like an epidural hematoma ala
Natasha Richardson, you'd be dead already.
There are other things that could be the problem. They could be caused by the accident, or they could bee unrelated. The therapist thinks it's physical (neurological), not psychological. And until your problem is diagnosed, it is impossible to determine what caused it.
It took my cousin over a decade to get a minor car accident settled, and part of the problem was getting the "minor" brain injury diagnosed. And the "minor" brain injury had a lot of long term consequences.
Aside: Even if you have insurance, a PCP may choose to ignore obvious neurological symptoms. Mine did.