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?
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.
Yeah, the ants may have the right # of legs; but did anyone notice that they are obviously the wrong color!?!? Geez... you would've thought they were making the movie for kids!!!
Every one is worried about the right number of legs and body color, but no one seems to mind the fact that they speak english, have play musical instruments, and build a scale model of a bird.