Why do certain people use swear words?

They are not intelligent enough to express themselves without resorting to foul language.

Assuming they don't have a lack of acceptable vocabulary, persons who seem to have an over reliance on swearing are often easily frustrated or agitated and this forces them to use 'shocking' language (aka swearing) to express themselves.

Usually, it’s because they lack the vocabulary to converse and express their thoughts without resorting to profanity.

Because the words do not carry the same emotional weight to those who use them as they do for those who do not use them. You question why they use them, they question why it bothers you. They are just words, after all. What matters is the meaning they are intended to portray, and not all words based on acts that are part of the social prohibitions against mentioning, are themselves used to mean those acts specifically.

In effect, you would likely accept the euphemism for a particular word but are offended by the more direct term. Both mean exactly the same thing if used that way, so both should be unacceptable when used that way. Yet they are not. You need to ask yourself why you feel that way, and why you think that a word, when used in a completely different sense than its original (and to you and most others unacceptable) one, retains that same sense of disapprobation. It is not used to refer to the disapproved act.

The open discussion of sexual activities with the intent to embarrass or otherwise discomfort an individual in the workplace is a violation of the law. The use of words that an individual finds offensive, however, is not prohibited, and is (in law) protected, except when that usage is done to present a racial or sexual context in a harassing fashion. The words are not the problem, the usage and intent is the problem.

I use colorful language. A lot of it is from the military. I don't in front of females and children.

I was talking to a ...get girl that looked like a guy. So I didn't think about my language.

After the 3 or 4th f**k(ing)(er) she looked at me and said

"I'm surprised you cuss so much. I thought you were better educated than that.".

IDK If that was calling me out. I watched how I spoke around her after that.

From an etiquette perspective it is not done to criticize other people's behavior unless you are their parent, teacher or employer. However you do not have to tolerate being cussed at so all you need to do is ignore them until they can speak civilly. Many people do it out of habit and are even unconscious of doing it but calling them out over it is just as rude.