There must be a good reason for this (London)

There must be a good reason for this .. What I am about to ask is politically incorrect but so many people do it that I am wondering if am missing something.

So hopefully someone intelligent and patient can explain it to me.

People who write something along the lines of 'sorry I can't make this one, stuck in the office, maybe next time' .. Who cares that you can't make it - you are ruining the vibe for everyone else and not adding any value.

600 people have RSVP'd yes.. No one is going to notice that you didn't make it.

Or maybe I am missing something and these people are so important that they need to let us know they infact can't make it or they look forward to making it to the event. Please enlighten me.