Because many people would get upset if "my-political-party" was hiring team of editors to change the wiki articles.

And if they are indeed getting paid, the organisation might have an important turnover. By using a single account (or multiple shared accounts; I doubt there's only one), you keep the 'prestige' you have built.

Your 1 week old new recruit can edit texts left and right while most people think it's a legit old wikipedians tidying stuff up.