Why don't all the symptoms of alzheimer's get better at once?

No one knows. Some symptoms can improve with treatment while many symptoms do not. Often the best treatment is offering support to the person and their caregivers. Since we still are not exactly sure what causes the illness it is difficult to identify treatments.
Symptoms. All through a degenerative process like alzheimers, symptoms will wax and wane, some improving, while others stay reduced. The best that our medications can do is to halt the slide, and or reduce the rate of worsening. It is frustrating for family members who do not know when their loved one will have a good day, or not be able to communicate coherently. Medication is worth the time saved.