Imo, I think parents are not teaching them to be polite forcefully enough. Sure they may tell them a million times to be polite, but it takes enforcement, and I always see parents "Letting" their kids not answer, interrupt, barge in front of people.
When my kids tried that stuff, they got clear explanations of WHAT they were doing and what would happen if they did it again. Even if I was unable to enforce then and there (in public), I DID do it after he fact as many times as it took and now they are all polite. They also have to ANSWER my husband and I respectfully, not just ignore. I know most of my friends do not do that. They call it kids being kids and make light of it and call it normal. It's a SAD trend. I can't count how many compliments we've had, and they almost make me sad, because it shouldn't be a big deal when a child uses commons manners. I always had to!
Once a ride operator at our local amusement park came up to me almost in tears to compliment my son (then three) because while she had been explaining his belt and to "hang on" he said, "Yes ma'am" and "thank you". She said she's worked in that park for seven years and had NEVER had a child call her ma'am.
Manners make people feel good. it's really sad people don't demand it from kids anymore. My kids get a lot out of being known as respectful and nice. It makes them feel good too. The only excuse to dodge the task is that it's difficult to teach.