Because the word has that meaning, a well-recognized meaning, and even if the speaker's intent is something else, that meaning resonates and registers in the minds of many hearers. I will persist in strongly disliking these expressions, while stoically accepting that there's not much I can do about it.
Discussion
If i'm not mistaken, 'fuck' is an angloid corruption of the German 'frick', from 'fricken', meaning "to strike". It received sexual connotations later, like so many other words.
Wow, that's interesting! The closest modern English equivalents I can think of are "bang" and "smash", though I'm not sure how those would work in an insult. Can you use frick or fricken in a sentence?
What if a word has multiple recognized meanings?
When a word has multiple recognized meanings, we have to determine which meaning is intended from context.
It's rather strange that in "f___ you" the message is one of extreme dislike, even hate, yet the actual meaning of the verb in this context is totally unclear, which I think allows a suggestion of the sexual meaning to always hover somewhere in the consciousness.
I speculate that this expression started simply as a means to shock, by using language that was at one time more or less forbidden.
A suggestion of sexual meaning is always bouncing around my subconscious regardless of the context. I think this case is pretty much closed for me, unless you think there's something we haven't talked about yet?
Ha. No, I suppose not. It's closed for me too. The negative expression "fuck you" (and all its derivatives) is a horribly regrettable development, though an unintentional one. It is as if a certain class of influential people had decided to make the word "love" also mean "shit," and had succeeded in making it the dominant usage.