The reason they might have is something to the effect of this:
Digimon are data (Digital Monsters) and created by humans -- while at the same time the Digital World existed all the while the human world did (don't ask, I'm not really a fan, and it was perpetually confusing as the first and second seasons had no, true ending).
Anyway, moving on, the Digimon are built upon programming that can understand human speak and, thus, learned it. I just take it as, "We're too cheap to come up with an actual story. Wait?! People like it more than we thought. We'll make it better for season four..."
Bah, that was pointless. Who really cares about Digimon? I'm serious, it may have it's moments but it's farther and fewer between than Pokemon.