My mother is like a zombie when it comes to being positive. Yet, she comes to life when anything negative can be expressed by her. The only time she seems to have any life is if she is being critical or judgemental of other people. Anything enjoyable she is involved with is met with completer indifference. She is not incompetent or diagnosed with dementia.
I think that even when doctors do see the early signs, they don't want to alarm or depress the patient. Imagine having to tell someone who's still pretty aware that they are losing their mind and there's nothing that can be done about it! Once the doctor knows the family is on board (meaning accompanying the patient to visits, the patient signing off that the doctor can speak to family members...) they can discuss your parent's status with you.
However, last night at family night when there was a slight problem with dinner I saw how some of the adult children there acted (those visiting) and was not surprised as the negative outlook on everything apparently runs in some families.