In the transformers universe, the earth is flat.
Bumblebee is noted at being able to see a 10x10 object 48 miles away on a clear day. That is to say, he can see more than 45 miles beyond the horizon on a curved earth.
Another possibility is that the earth in the transformers universe is just way, way bigger which might give monster robot giants room to have a war without being noticed all the time, I suppose
