Why do some people think it's ok to break plans at the last minute?

After moving to a new country and establishing a new social circle, I've noticed a trend in these people repeatedly making plans, and breaking them off at the last minute. I've only noticed it since I moved, and I'm wondering whether it's to do with a...

country's social values, whether it's something that is just accepted in a certain age group, or whether it's just these people in particular. Maybe it's just me, but I brought up to understand that breaking plans at the last minute was incredibly rude, and only to be done in an emergency situation (as in, life or death).

I'm interested in finding out other people's opinions- is this a cultural thing in some countries, is it just a certain age group, or is it just some people with no social graces?

I think sometimes people say that they'll do something because they don't know how to say that they don't want to. Then, they cancel at the last minute with some stupid excuse.

I really don't know how to generalize the group -
I've had this happening to me for a couple of years now actually, friends making plans then all of a sudden dropping them. I've actually befriended a couple of people because it got to the point where it was happening so much I couldn't stand it anymore.
I guess some people just don't care about plans and don't think it's a big deal, I do, if I have plans I make an effort to prepare and usually look forward to them.
I totally understand if an emergency comes up or the person is sick, but that doesn't ever seem to be the case and it pushes my buttons.