My thoughts. Not reading the rules aside, it's mostly because of one or a few things:

1. They don't think it's a big deal. A lot of people can post copy-pasted content on other forums and communities and it wouldn't matter that much. (not saying that it's fine, because it's definitely not)

2. They didn't think they would get caught. Like, I honestly think we're only one of a few non-professional communities/forums that are strict with plagiarism lol (which is a good thing). Unfortunately a lot are just focused on getting merits and ranking up the easy way.

3. Just laziness