It's because there is little progress in uncovering the mechanisms of the disease. Alzheimer's et al are tau-opathies and Parkinson's et al are synuclein-opathies. Both are degenerative and we wait for their common base in a Grand Unified Theory.

I like to view degeneration of any tissue as death due to (premature) aging. So, if we treat it, we achieve immortality by applying it to all body. This is something hard.