Why was this accepted?
Because not everybody Googles the text of suggested tag wiki edits before hitting the “Approve” button. That doesn’t make it okay; it just means that those reviewers didn’t do due diligence before approving the edit.
I believe this is the sort of problem that review audits are meant to solve, but I don’t know if they’re enabled on SFF or whether review audits would try to catch people for this sort of plagiarism.
The Meta question Proposed methods for countering plagiarized text in tag wikis has some interesting suggestions about this topic if you’re still interested.
Would an edit be valid if it was copy pasted from Wikipedia and then mentioned that the citation is from the Wikipedia?
It would be valid in the sense that it’s not illegal and shouldn’t be rejected as plagiarism, but it wouldn’t necessarily be good. It would be factually accurate, but that doesn’t make it useful for readers of our site.
See Gilles’s answer to Is copy/paste from Wikipedia allowed on tag wikis? or point 2 in Keen’s answer to Why are my tag wiki edits rejected? for a longer explanation of why Wikipedia excerpts don’t make the best tag wiki edits.