Thing is though, for the most part, people used to socialise with those in the same neighbourhood, or club, street or small town. People live a long way away now and have busy lives with two full-time working parents being the norm.
The days of popping to a neighbours for a BBQ are dying out. Our most successful parties have always been fairly impromptu, so same week notice, and for people who are around our house/neighbours/friends who are less than 30 min drive away, everyone bringing something to put on the BBQ. None of my friends socialise in advance much any more, or at least, if local, it's same week/next week possibly- we just see who's around on the weekend and go from there.
Far less successful are things arranged in advance, require hotels, certain outfits etc, unless they are a wedding or funeral.
I think everyone saying 'everyone else is flaky' might just take some time to think if they always attend everything any more and if not why not.
I know why I sometimes cancel at the last min, and so do my friends as they know my life and how unexpected some of the things are that happen within it (due to children).
This season is also full of bugs, 'freshers flu' is going around, Covid is rife and I've had it so expect some people to cancel either because they are ill or their children are ill at the last min.
I don't think there's some sinister 'flakiness' going around, I think people are tired from work, exhausted from caring from their kids and have less disposable income, plus it's cold season- party planners plan for up to 50% drop out and I think if you have a few good friends in a room for your birthday you are doing well.