Neurons in the cerebral cortex are not replaced when they die.
The body does not actually replace its cells every seven years.
If it did the question would be: why aren’t we seven years old forever?
The question really is: why does it make worse replacements every time?
Cells are replaced, always with the same materials and instructions.
Except the instructions instruct the replacements to be gradually inferior—
the instructions deliberately lower our quality with every cell swap-out.
Our bodies are telling our bodies to wear down and to fall apart.
Brain cells just keep dying.
That explains a lot.