Why do I have symptoms of dementia but a CT scan showed nothing?

Not sensitive. Ct is insensitive and nonspecific for dementia. Mri and pet are better, but clinical evaluation by a neurologist is most important.
Anatomy vs function. A very advanced dementia would likely show cerebral cortical atrophy and enlarged ventricles. Earlier in the process you might not see anything at all, though -- because a ct scan registers only anatomic changes, not functional or physiological ones. These may go on for a long time, and could also result in symptoms long before anatomic changes show up on ct.