People think they have something new to offer, because they cannot believe anyone else may have thought of that before them?
People write something and then forget to post it for ages (I do this!)
People - and I genuinely think social media can take the blame for this - have almost been conditioned to just blart out their thought without reading the full thread, which I liken to someone walking into a pub, hearing a snippet of conversation, shouting their response over the top of everyone else and then walking out again.
Facebook doesn't help, and nor does the use of mobile phones, with limited views of threads and it often being hard to find the start of the thread on Facebook.. and then people find forums after they've got used to facebook and treat them the same way.
In the good old days before facebook, most people on a forum would read the full thread, but then, most of them were using a laptop or pc, so it was easy to see a lot of the thread whilst replying. These days thats not so easy on a tiny mobile screen.
It's also much more likely to happen in a group where there isn't much of a community feel, everyones anonymous, no one knows one another.
Again, back in the day on small forums where everyone got to know other users reasonably well, there would be much more interest in reading the full thread to see who had replied and what they thought about the topic.