Why is the epidural space absent in the brain?

In the spine, the epidural space (also known as "extradural space" or "peridural space") is the outermost part of the spinal canal. It is the space within the canal (formed by the surrounding vertebrae) lying outside the dura mater (which encloses the arachnoid mater, subarachnoid space, the cerebrospinal fluid, and the spinal cord). In humans the epidural space contains lymphatics, spinal nerve roots, loose fatty tissue, small arteries, and a network of large, thin-walled blood vessels called the epidural venous plexus.

The upper limit of the epidural space is the foramen magnum, which is the point where the spine meets the base of the skull. The lower limit is at the tip of the sacrum, at the sacrococcygeal membrane.

In the head, the dura is continuous with the periOSteum, the tough fibrous lining of the inside of the skull. This means that, in the head, the epidural space is known as a potential space, which means that normally it does not exist. In rare circumstances, a torn artery (e.g. the middle meningeal artery) may cause bleeding which is sufficient to separate both the dura and periOSteum from the bone; this is an epidural hematoma.

The space between the dura and the arachnoid (in both head and spine), the subdural space, is also a potential space. Bleeding may also occur here.

reference: wikipedia