Elaborating a bit - brain is hard to study since you can't easily take a biopsy of it (from a living person at least), and various brain scans are not great at identifying the stuff we care about.

The slow acting nature of it means also you have to wait a long time to see results of clinical trials; also because early stages are easy to miss that also means you are stuck studying people who are already pretty senile and thus might be beyond the point where you can make a big difference.

Ruxandra has a nice piece, focused on cancer, but the reasoning is basically the same here: biology is just really hard. Sometimes we get lucky but in general it's a long, slow slog.

[1] https://www.writingruxandrabio.com/p/why-havent-biologists-c...

And as with a lot of cancers, it seems to be perturbation of a dynamic system rather than a single, mechanistic cause.

Think of it like brushfire in an ecosystem, or species population imbalances leading to catastrophic breakdown. These are better understood in terms of system state and preconditions, as opposed to a trigger event.

Infectious disease, at least in the classic acute form (whether that's bacterial and fast - cholera - or viral and slow - HIV), is a more mechanistic process which can be halted by blocking a single step in its pathway.

Systems that remain healthy and balanced via dynamic processes are harder to reason about and fix, because the root cause of a disease state can be dozens of little things adding up to the system losing its ability to maintain homeostasis.

You can't definitively diagnose it without an autopsy of the brain.