Because of almost certainly fraudulent science. See https://www.science.org/content/article/potential-fabricatio.... Research was misled for decades down the path of assuming that Amyloid plaques are causative of Alzheimers, a now disproven theory (there's an association, but that's not the same as causative.)

Scientist Ruth Itzhaki spent years studying a far more promising theory of Alzheimer's: that it's caused by viral infection in the brain, particularly HSV-1, best known for causing cold sores. Most have it, so there are clearly other factors at work, likely related to susceptibility in particular individuals to to the virus infecting the brain and spreading over time. See https://pubmed.ncbi.nlm.nih.gov/34205498/

The implication is that anti-viral treatments are likely to inhibit and potentially cure Alzheimer's. There is already unintended evidence along these lines, both via antiviral drugs and vaccines.