I can't see why the writer uses but not so. Does "but" mean "so" in the context? It seems to be clear to me that A (I'm getting older) is the cause of B (every time I learn something new, I forget something else).
"but" is there because of the wording "maybe it's because". "Maybe/perhaps/possibly/etc. it's because/owing to/etc.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
"but" is there because of the wording "maybe it's because".
"Maybe/perhaps/possibly/etc. it's because/owing to/etc. X, but Y" is a set pattern of words that we use when Y could be a reason for X.