You can't really blame the healthcare system for this. Alzheimer's and Dementia existed before modern medicine. The reality is that many fit, active, and otherwise healthy people will hit their 60s and 70s and will experience cognitive decline and Alzheimer's.

They hit their 60s and 70s because the health care system is good at fixing certain physical issues not bugs accumulating in the brain. The brain just like your OS cant just keep getting patched forever. So currently people just keep patching older wearing out hardware without any software upgrades available.