I think it's because family and friends think/fear/worry that if they admit there is a diagnosis of Alzheimer's, it will obligate their time, their money, and/or their home if they choose to help

, and will make them feel guilty or ashamed if they choose to be no help at all

. Worst of all, for family it may mean that they might get Alzheimer's, too.
Simple DENIAL frees them of all those bothersome worries and feelings!
As Mark Twain wrote, "Denial is not just a river in Egypt!!"
Cheers! Barbara