It isn't. I would read up on how the Fed was created. The choice of cities was political (that is why Missouri has two, iirc) and related to the situation in 1913 when the Fed was created (in particular, the cyclical demand for money from agriculture).

Now, it makes almost no sense to move them. After 1935, the Banks have had fairly limited power.

What changed in 1935?

Reorganization of the Fed. Most importantly: Board of Governors created in Washington. Combined with the exigencies of WW2, this basically finished Reserve Bank power. And this structure did worsen the Great Depression (Reserve Banks acted independently and wouldn't participate in market operations).