Do sundural hematomas ALWAYS need surgery

if not, why not?

NO.
It depends on the acuity of the bleed and the size. Acute subdurals may require surgery if there is significant midline shift and symptoms. Chronic subdurals may require surgery if they are very large and causing symptoms.

Small subdurals which are not causing symptoms do not require surgical evacuation. The hematoma will resolve on its own with time. The person should be monitored in a hospital for several days to make sure the bleed does not increase.

Small subdural hematomas may slowly resolve themselves. If they are not causing pressure symptoms they can be left alone till they get organised/fibrosed/calcified.
Or else surgery is needed .