Cough
As Lyndor said, completely different situation.
You make an appointment, on the day it occurs you plan to ensure that you have ample time to atend in a timely way.
You receive an invite - usually not when you are conveniently sat at a desk with your whole family's calendar, and the leisurely time in which to address the matter. Unless it's a physical invite, it either has to be carefully left as unread, or otherwise brought to your attention when you CAN answer if you're available.
And quite frankly, we are talking about invites to kid's birthday parties. They slip down the agenda because they just AREN'T as important to the parents of attendees as commute-job-ironing-doctors appointments-bills-calling the bank-dropping off mum's birthday card etc, etc, etc. And that includes a courtesy reply to say you're thinking about it/checking.
I do bloody lots of things, and I don't even have kids. I'm not particularly concerned about leaving a low-priority invite to wait a little whilst I enjoy the half hour I have free each day. If people are going to be sensitive about that they're not cut out for friendship with me
.
I mean, bloody bravo and I'm chuffed for you if you've got your life organised and so on, or have little enough to do, or this is just a super huge priority for you. But you might be happier to accept that other people see it differently.