There are four reasons, actually. You left out to scam people" and "to get fast cash by outright plagiarism".
I honestly don't know what to do about the spammers. It would be nice if we had some actually active site admins that would help the community with these issues instead of just leaving it to the community to deplete their voting power over flagging these spammers.
There are people doing something about it. Check out the following posts:
This isn't the entire answer, but it helps. https://steemit.com/spam/@personz/i-run-sadkitten-so-you-don-t-have-to-dealing-with-massive-amounts-of-spam https://steemit.com/spaminator/@spaminator/10-102-spam-comments-need-flagged