For the same reason, it's not the same thing as saying "Oh, shit," when you realize that everything just went way down south for you, and the next several hours, if not days, are going to be fairly stressful and difficult.
The issue isn't word choice, although if the range of your vocabulary is limited to the F-bomb, you desperately need a dictionary. It's the thought and intent behind the word that determines whether language is filthy, or not.
When Jesus talks about righteousness, it's clear he's concerned with what's going on in a person's heart far more than with the externals of what they are doing or saying. A person whose vocabulary is limited to expletives may not be fit for polite society, but I wasn't aware that polite society is what we hope to be fit for.
For a rather tongue-in-cheek essay on this, take a look at Toward An Evangelical Theology Of Cussing.