If I'm writing a letter to someone I don't know very well, I might begin it "Dear _____" and end it "Yours truly." But nobody is under the slightest impression that the recipient really is dear to me, nor that I'm declaring any sort of fealty.
I said "nobody," but of course that's not quite right. Nobody who's even noddingly familiar with the conventions of letter writing will be confused, though someone from a very different culture might be. What someone means by using certain words isn't just a matter of what you find when you look the words up in a dictionary.
Or suppose I run into a nodding acquaintance by chance. I hug them and say "Good to see you." Is the hug an expression of intimacy? Am I really pleased to see this person? Maybe or maybe not, but at least in my part of the world, this is how people great one another. I don't make judgments about people's overall sincerity based on interactions like this, because in following the conventions of polite greeting, sincerity isn't the issue.
Do conventions like this really undermine the usefulness of words like "good?" I'm not convinced. There are all kinds of contextual cues that help us figure out what people mean, and typically we pick up the cues more or less automatically. For example: if I'm having dinner at a mutually-agreed-on restaurant with a friend and he spontaneously says "This risotto is really very good!" it's a fair bet that he means it.
Is it always easy to tell? No. Are people sometimes insincere in social situations? Yes. Is this a bad thing? Not necessarily and certainly not always. We have to interact with people we like and people we don't like. I may not like John, but there may be no good reason to rub his nose in that fact. None of us likes being snubbed, and often there's nothing to be gained by putting our true feelings on display.
We use words to state facts, but we use words for many other things as well. Social conventions and forms of politeness do something important: they help us get along, sometimes by papering over differences. By and large, getting along is good. Often it's at least as important as saying exactly what we think.