Why is there even such a thing as foul language?

Today at work I rhetorically asked a coworker, “what the hell were you thinking,” only for a bunch of coworkers to give me a glare and sternly warning me to not say hell again. Having just moved from California to Alabama, I was sort of taken aback having only ever been slapped on the wrist for saying hell by nigh fanatical individuals of Abrahamic faith back in California.

Anyway, it got me to start wondering why is there even such a thing as foul language in the first place. There are plenty of words and phrases that can be/are offensive which aren’t considered to be foul language so why the restriction on just a particular set of words?