To do that you first need to distrust AI, and a lot of people don't. They think of GPT like Google-but-written-in-English. That is a large part of the problem.
That's not a valid excuse, though. Lawyers are paid big bucks to think, not to assume. Otherwise you could do your litigation for free by just asking interested people on Twitter. I went to law school and had to drop out due to an injury & attendant medical costs; it's a crime (as in going to jail) for me to practice law without being licensed, no matter how good my work product might be.
There are probably thousands of lawyers that thought about using ChatGPT for their profession, most of them realized it lies and never got farther than that, maybe a few hundred actually tried it out and also realized ChatGPT was lying, this is the one guy managed to swiss-cheese-model his way through.
Well, leaving aside whether a lot of people don't distrust and whether a lawyer would or should be among those people, lawyers typically don't cite things without checking them first. I work at a biglaw firm, can't imagine it never getting caught that the cases don't exist. This sounds like a midsize firm, should be fairly similar.
There are a few small law firms/sole prop type guys, however, who I have crossed paths with for whom this kind of stupidity and carelessness would be on brand though.
Guess he was just in a rush and figured this would be one of the 2/10 times he files something without at least taking a look at the opinions first, and it ended up being a massive error.