Yeah, I know what you are saying. You are right, its a nice thing to do for the other person. It just doesnt occur to many people and an engagement card was not something I was hugely aware of myself so when I read about various situations that people are upset not to get this or that, I feel a bit freaked out that I've probably upset a friend etc. through my own ignorance of what is required to mark whatever occasion. I also think that people are busy in their own lives and often just dont get around to it and that doesnt mean they dont care enough, they just have different opinions on what is needed or fail to understand how important it would be to the receiving end.
All Im trying to say is dont be upset. Its an important time for you but everyone is going to react to varying levels. It might just be that they had other things on their mind the day you told them and they forgot to get a card until they felt it was too late. Maybe it just doesnt seem like such a big deal to them because they saw it coming for the last 3 years....who knows why people do or dont do things but when you are talking about friends or family, its rarely for a negative reason (discounting all the crazy MIL's and SILs etc that people have posted about on here that genuinely sounded like they were trying to be nasty).
Having said that, in the interest of 'live and let live' you are entitled to be annoyed about anything that annoys you.