K.:
Hello! I know I will probably get reemed for this answer but it's my opinion.
This shows the society we have become - lazy and self-centered. We've allowed it to become this way.
Most parents are busy working and don't take the time go through their kids backpacks (if they are handed out at school) or they just "assume" that you will know they will be there.
When I send out invites - I give people two different ways to contact me - e-mail and phone. I also ensure I have their e-mail addresses as many people just claim "they didn't get it". And I send out "save the date" e-mails giving them specifics and telling them to look in "Jack's" backpack or in the snail mail for the invite.
Just like Thank you notes - when was the last time you got one of those? I make my boys write them out. PERIOD - no thank you note - no gift (I don't let them play with the gift until a thank you note is written)