Many Open Source licenses require attribution. Here is a summary of some of the popular ones attribution requirements: https://www.nexb.com/blog/oss_attribution_obligations.html

As you can see here, plagiarism is not inherent to Open Source, and often times it is barred by the many of the most popular licenses.

I am sorry, what?

Open source software license does not generally allow such thing.

I know we won't be able to get an answer to this question unless it actually goes to court (and I haven't seen anything about this any court), but couldn't you take any MIT licensed project, replace the license with your own and it'd be legal? My understanding is the license means you're allowed to freely modify anything related to the project (unless someone has a trademark/copyright of course)

The MIT license actually has one requirement:

> The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

Attribution (In the form of this requirement) is the one thing the MIT license actually requires.

The GPL variants, for example, require you to allow others to plagiarize your work.

The GPLv3 specifically requires copyright notices to be kept in tact on verbatim copies. Other popular Open Source licenses have more stringent attribution requirements. Plagiarism is definitely not inherent to Open Source, though it may be inherent in some licenses.

No. As far as I know, it doesn't.