As someone who used to think that cussing was bad, I didn't understand why people did that too. But now I cuss when I'm angry or frustrated. Of course it doesn't help the situation but it actually does help some of the stress. It's kind of like a form of catharsis. Not everyone feels this way, but it's quite common to do so.
Cussing also adds emphasis. I know there are other ways to add emphasis but because cussing is taboo, it appears to add more emphasis to the situation or the word. I personally don't do this (unless I'm mad) but if someone adds a cuss word to something, I know that they really mean it.
About looking stupid, I think that if people cuss in almost every sentence is when they start to look stupid. It's as if they don't know how to say a normal sentence which make them appear as if they're uneducated. On top of that, saying cuss words in settings that doesn't seem appropriate like during a funeral, a wedding, or a business meeting would also seems to signifies a lack of education. But saying it occasionally in a casual setting doesn't really make people look all that stupid in my opinion because they're still very much capable of saying normal sentences (and they usually do) and they are intelligent enough to know when it is appropriate and when it is not. But if you still think that they're stupid then that's up to you. Just don't say this out to anyone who does cuss as a way to insult them or else they will brutally come right back you (I had to learn this the hard way so trust me).
I know that you hate hearing cuss words and I know the feeling very well because I was once in your shoes. But you honestly cannot change the way people are. That is something that I always see with other people is that they try to change how everyone around them behaves in order to suit their own wants and needs. I will be brutally honest and say that this type of mindset is very unhealthy because it is impossible to achieve such thing. There will always be things and people in this world that we don't like and that makes us angry. And if you are constantly trying to change others, your goal will never be achieved and you will become frustrated. I know this from personal experience.
So if this is a serious ongoing problem for you, you can politely ask them to not cuss when you're around. Don't tell them to stop altogether but just ask them to be considerate whenever you're around them. When you ask them, don't be mean about it. Be calm and tell them politely that you are not comfortable with them using those words and would highly appreciate if they don't cuss in your presence (or if you were like me at that time and capable of beating the crap out of them, that works too although that is highly not recommended). If they are truly good people, they will understand. If they don't, then just leave the area and don't associate with them. But if for whatever reason that is not possible, then this is a good opportunity to learn to tolerate or ignore others. Not everyone can adjust to your needs so sometimes you need to adjust your approach in order to get what you want. I'm not saying that you need to suddenly love cussing. I'm just saying that you just need to shift your focus on other things in order to not let it bother you. Over time, I've grown to be able to ignore and tolerate almost all people and things I don't like so now I'm usually all right being around them. It takes a lot of time and practice, but I believe that learning how to zone out the things you don't like is an important skill to have if you don't want to cause conflict with other people. You don't have to love it; you just need to tolerate or ignore it enough that it no longer bothers you.
Sorry, kinda went on a tangent there haha! Anyway, you should also check out the video below! I think that you would appreciate watching this video to help answer your question. Also Vsauce (the YouTube channel) and Michael (the guy in the video) are awesome!
Okay, I hope this helps!