I also don't have kids, but I think a lot of it has to do with the fact people are just ruder than they were, and place less value on etiquette. I know i'm generalizing, but I'm amazed sometimes at how people act, compared to how I act, and how I was brought up.
I also think the "must invite the whole class" thing has a lot to do with it as well. I know my friends with kids seem to have an endless stream of birthday party invites, and with more parents working, and less free time, I can see how it could get overwhelming.
I'm trying to remember my birthday parties; sadly I didn't have a lot of friends, and I don't recall even having a party every year. I can remember one or two, and maybe had 5-6 friends invited and come. As far as being invited, I don't recall being invited to all that many, esp once we moved when I was in 4th grade. I think I can honestly say I didn't attend any birthday parties from that time through 6th grade. I know kids had them, but as I had few to no friends, I wasn't invited.