Why are banks in America designed like fortresses? Is this necessary?

For the most part, they aren’t. But, there are holdovers.

Basically, a large part of the banking system grew up during the “Old West” period where the US was expanding. All exchanges were physical money except for loans which were recorded.

So, bank buildings were designed like secure boxes. Better ones were built like cages. The best were built like vaults. As a result, few people robbed banks, but mostly robbed the runners shipping lots of money to various places.

In New York some of the best of these examples still stand, and Hollywood loves them. Giant stone buildings, guards, ornate gates and cages, visible security like cameras. Most of it meanless. This is all show to make customers feel like their money is protected.

Simple answer. It used to be for protection, but now people still like the feeling that their money is protected.