I think the rise of mobile phones has massively contributed to this.
I remember almost 20 years at uni going out without my phone one day and I was supposed to be meeting a friend but she didn't show up in the agreed place so after 15 minutes I left. When I got home there were messages from her saying she was going to be late, then messages asking where I was, then missed calls. I was just like, "I didn't have my phone on me so I waited in our agreed meeting place and when you didn't show up, I left." And she was like, "But WHYYY didn't you have your phone on you?"
I don't know. I just didn't. But I made the point that in pre mobile days people just had to be where they said they would be, when they said they would be there.
I'm now way more attached to my phone than I was 20 years ago and would always see a message unless my phone actually got lost or broken. But I think the principle is still good. It's so easy to contact people, and we expect them to read their messages instantly, that it's too easy to cancel at the last minute when we don't feel like doing something.
These days I keep my friends list purposefully small and there aren't any flaky people on it.