The same reason the artists give all cartoon characters 4 fingers. It's because it is easier to animate and looks a lot tidier. As it is a kids film, they do not notice. The artists always take the easiest and nicest looking way they can.
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They wanted to keep the drawings simple, they figured that a million ants with two extra legs would cause a lot of problems. Besides, the ants needed two arms for the whole point of the story, know what I mean?
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