Jokes may be fun to write. But when you're scouring a manual for a needed function, and you're confronted with a smarmy joke, it probably won't be as fun (especially if you're reading the manual to troubleshoot something.)
Back then manuals were also far more important, you couldn't operate the machine at all without one. Yet the jokes never managed to stop people from getting the information they needed.
I think the real difference when it comes to manuals is most modern ones are not comprehensive. I could see being upset if there was a joke but no information on my actual problem.