Don't worry, we have now identified tau tangles. This must be the root cause, since it is unfathomable that a complex neurodegenerative disease could have complex causes. Now that we have identified [insert misfolded protein candidate here], we can finally create a mouse models with the protein broken, then fix the protein, and then announce a cure. /s
Just so that this comment has something constructive in it - I'll add that I think it's important to understand that scientists are human and are also drawn by the allure of oversimple explanations for complex issues. For alzheimers, of the cause was a single protein or gene or something we already understand, we would have found it already. This isn't some ultra rare disease where nobody is looking. Its hard to say "I dont know," but that's the truth. The answer to this is going to come out of some obscure basic research that doesn't even seem obviously tied to neurology yet - and whatever team is working on it is going to have no idea how important their work is and is still going to get a great "I told you so!" moment out of it with regards to all the people asking them if their research has any practical purpose right now.
Just so that this comment has something constructive in it - I'll add that I think it's important to understand that scientists are human and are also drawn by the allure of oversimple explanations for complex issues. For alzheimers, of the cause was a single protein or gene or something we already understand, we would have found it already. This isn't some ultra rare disease where nobody is looking. Its hard to say "I dont know," but that's the truth. The answer to this is going to come out of some obscure basic research that doesn't even seem obviously tied to neurology yet - and whatever team is working on it is going to have no idea how important their work is and is still going to get a great "I told you so!" moment out of it with regards to all the people asking them if their research has any practical purpose right now.