If writing good commit messages isn't specifically defined as part of your job, why would you waste business hours writing commit messages that are beyond what is expected of you and frankly useless since nobody would ever read it anyway?
I wish github allowed to make the copying automatic and ensure that it happens (it doesn't, unfortunately).
If someone wants to learn the full history - the remarks during code review, perhaps all the WIP commits - they can read the PR/code review comments. I found it to be very rarely needed.
All of our commit messages were nice, long and detailed, with a link back to the PR if you really wanted to go back and see the individual commits and/or discussion that occurred on that PR. I think I only looked at individual commits maybe once or twice since they were usually useless in isolation (woops, WIP, fix typo, etc.).
No-one's going to ALSO write commit messages that no-one will see.
Not sure whey e.g. Github Pull request merges don't include the entire pull request description in the merge commit message.
Helps with reviewing as well as blaming.