why isn't it hebben? since 'everyone' implies multiple people.
In English, the words "everyone" and "everybody" certainly imply multiple people, but they also require the verb to be conjugated in the 3rd person singular.
everyone/everybody has = iedereen heeft
I also found this explanation (http://www.dutchgrammar.com/en/?n=Pronouns.Id03), which is part of a pretty detailed list of Dutch indefinite pronouns and their uses.