Among the many illogical aspects of an endeavour that should be driven by pure logic and cold data is the assumption that compulsory medical examinations are the primary way in which risks caused by medical conditions are 'trapped' and, conversely, that having identified some 'exclusionary' condition in a person, the person is somehow prevented from flying. How is someone with dementia capable of remembering and carrying out all of what's necessary to commit aviation? Assuming someone with dementia has enough remaining capacity to be dangerous, how does identifying their condition (and even revoking their medical certificate if they have one) of itself stop the person doing dangerous things with aircraft?
The substantive 'trap' for risks caused by medical conditions is personal responsibility and checking and training and flight reviews and colleagues and family and friends and other contextual factors. An observant GP would indeed pick up a patient with dementia in a short conversation, but in most cases the GP (or DAME) will be the last person in the patient's life to find that out. In circumstances in which those contextual factors are lacking, no medical examination or certificate revocation is going to stop someone from flying if they are minded to, whether due to demented confusion or clear headed intent. There are people who go flying, deliberately, without the medical certificate required by law. Most of them survive, and the ones that don't survive usually spear in through a lack of competence rather than some medical condition.