Yeah, but I think there was a good reason they never revealed themselves. And that is that they are more advanced than anything the Andalites or the humans have ever created. Not only does this mean that everyone would want to dissect them to see how they tick, but a lot of that technology could be used for violent purposes. Maybe at first, everyone is just like, "Hey, cool! Super-intelligent robots!" But the Chee probably know perfectly well that, sooner or later, someone would wonder how to use that kind of artificial intelligence to design killer soldiers, and the Chee (quite rightfully) want no part of that.