Why is the bank icon typically designed the way it is?

The icon is a front-replica of the US Federal Hall, and pretty much how most banks followed this architecture style. The US Federal Hall served as the Sub-Treasury of the US Government (in addition to being the first capitol building in the US).

You can read up more about the Federal Hall (in New York City) on Wikipedia: Federal Hall.

Also see: National Memorial


Most banks around the world followed and utilized such an architecture.

This is how the building looks like today:


and another view...