If they required 2FA every time you wanted to modify JS then it couldn't propagate automatically. Just requiring 2FA when you first log in wouldn't help, of course.
2FAs also may require a level of KYC that Wikipedia isn't after and advocating for 2FA might indirectly advocate for a lot more things than just 2FA.
More to the point, if they required 2FA every time you tried to modify the JS, nobody would do it because it would be too annoying. "Username, password... oh, the 2FA just timed out, gotta wait for the next one... what, that doesn't work? Does it want the old one? Oh... now it wants the next one... just a second... "