Quote Originally Posted by Inf View Post
Because it's a whoop, not a verb.
In many cases (if not in most) "поехали!" used as "let's start".
"Whoop" is not a grammatical term, if not anything, the word is a verb.
Verbs in past form like поехали, пошли, побежали, полетели are used either to describe started actions in the past or to urge to do the action at the moment.