I know what you mean. Often, the clerk doesn't say hello and simply starts ringing up your groceries acting like they are mad at the world and pissed off and angry.
Then, after you pay them a bunch of money they don't even say "Thank You!" or "Have A Good Un!" Many times, to break the awkward silence I say "Thank You" and often they will respond half-heartedly with a "You're Welcome" If I don't say Thank you, then it feels weird walking out the store with no Pleasantries being exchanged. I think that they should cheerfully greet you with 'Hello, Welcome To Safeway! How Are You Doing To Today, or Did You Find Everything That You Need?" and then when they are ringing up your purchases they can say stuff like, 'I see that you're buying strawberries again!
Oh, you're so predictable!" or 'Nice weather we've been having lately!" or just make some kind of joke or whatever. It is unfriendly the way some of them don't say a word and don't say thank you or nothing.