The two are both medium of exchange but they do not have exactly the same structure. I can easily convert bitcoins into money without leaving the house, unlike dollars before I convert it into fiat in my country I need a money exchanger. Bitcoin is digital currency and intangible asset while dollar is controlled and issued by the government and it is a tangible asset. They are both can be used as payment but bitcoin is in a more innovative way.
Many banks offer multi-currency accounts which allow you to effortlessly convert between different fiat currencies without leaving you home and with minimal spreads. It looks like you are mostly talking about exchanging paper dollars but since there are no paper bitcoins, your comparison is not a valid one. In most cases you can convert dollars (or whatever fiat you have) in your bank account without much ado, so there is no difference or it is only a small and insignificant one.