I had the same thought. My friends and family are all mostly in the same income bracket, and "don't have the money" would be an obvious lie for all but the most expensive event (recent example was a luxury safari in Africa at >$10k/head).
I use the 'dont have the money' excuse all the time for some people. As the other side seems to be a consistent lack of respect for my time, for some people. 'what why? you are not doing anything anyway'. Basically they have decided whatever I was going to use that time for is better spent with whatever activity they cooked up. Sometimes that is true. But the money thing seems to get them to think it through. The impulsive people seem to get it more, as they are usually short no mater how much cash they make. They seem to find a way to burn through it. The more thoughtful people approach it with a respect of my time and I can discuss it rationally with them.
I wonder if there's an age component. I'm mid-40s. There's zero expectation that people make late-breaking plans to go party downtown (as would have been the case in my 20s). And some friends have kids, which just complicates things more. At the end of the day "nah, I have [life] going on" is a completely normal response.
You could, but that really isn't any different than "I don't want to come (for some other reason)."
There's a difference between "don't want to" and "can't do".
But, we have a pretty tight-knit group of friends, most of whom live within walking distance, and due to kids, we usually just have dinner at somebody's house. We're boring white collar suburbanites, I guess.