The facts are these:
1) Signature campaigns/bounties most often pay for posts in English.
2) Many participants in those campaigns/bounties are non-English speakers.
3) Many are also dishonest and lazy and think nothing of trying their luck with the rules, knowing they can just create another account if they get caught, even if that itself is against the rules.
That's the long and short of it. Also, I would suggest that we don't need an in-depth analysis of why people plagiarize on this forum. It's not rocket science.