Besides, fully representing the heavy fighting vehicles awesomeness in combat in a game is not an easy task. A 20/30/40mm cannon can't take on a tank, and is pretty useless against infantry, at least in a game due to the low rate of fire.
For the IFVs to be any good, you'd have to be able to punch holes through walls and other cover, which is kind of hard to implement. Otherwise, small caliber cannons would just be slow firing MGs that can knock out tank tracks, not nearly as effective or cool as in real life.