I didn't say subjective content, I said content about something subjective. Wikipedia contains a wealth of "one proposed explanation for", or "a common theory is" on any event or phenomenon, (of which many are covered). Objective reports of subjective statements. And the choice of which to use, which perspectives to include, is a form of bias. The reporting of which proposed theories for causes of historical events or meanings for literature are included, and which are left out, is a form of bias. One that cannot be seen through simply by "checking the sources". An article written with a slant is going to include sources that agree with its viewpoint and not include sources that do not, and checking the sources is going to show you those viewpoints, and not the ones that were left out.

Also, again, there are absolutely editors who will just wordlessly revert objective, factual edits, with clear, proper citations from accepted primary sources, just because it's their page or it doesn't line up with how they want it to be seen. Checking the sources won't show you that, either.