Please explain reasons why the term glioblastoma used to describe all brain tumors if all brain tumors aren't composed of glial?

It is not. Glioblastoma is not used to refer to all brain tumors - this refers to a single very specific tumor that arises from glial cells (astrocytes) and is the highest grade of Astrocytoma. There are numerous types of primary brain cancer , all with different terminology that generally reflects cell of origin (Astrocytoma, oligodendrogliomas, medulloblastoma. Meningioma, Ependymoma, and etc)
Not true. Gliomas are brain tumors arising from glial cells. There are benign forms, anaplastic types, and then there are aggressive glioblastomas. There are many other, less common tumors of the brain, but they have different names and are not called glioblastomas, generally or specifically.