Many wikipedians are obsessive types. Also why would multiple people pretend to be one. Groups are more effective than individuals on Wikipedia.
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.