Sounds like that person that sent the " I see you didn't invite me.." is harboring some existing issues. When people send me stuff like that, I take it that I've offended them in some way previously that I, personally, am not aware of. Still, that's his/her issue..not mine. And a message like that is a surefire way to make sure you NEVER receive an invite from me...for anything...ever!
And you're right, people need to stop assuming that they'll be invited to every event. There's a lot of competition out there and you may not be the best fit for the "job".