What causes glioblastoma in brainstem, & high grade glioma in brainstem in young adults?thought brainstem had no glial cells to cause this in adults?

It's just bad luck: There are a few genetic syndromes that increase risk for glioblastoma / "high grade glioma", but beyond this it's simply a misfortune. All cancers arise from random mutations -- these are totally dumb luck. Anybody who told you that the brainstem has no glial cells is somebody you either misunderstood or simply shouldn't listen to. It's got a lot more glia than neurons like the rest of the brain.