Hi Phil. The reason for this is because "on va" in French already covers the English progressive present tense of "we are going." It's a situation where, unfortunately, you can't translate directly from English to French. In fact, "on va" can mean we are going, or we go, depending on the context of the sentence, and all verbs in French work this way in the present tense. (And pretty much every other tense, but there are helping verbs in this language.) If I say, "Je mange" it means both I am eating or I eat. I hope this helps!