There's also a more important concern that deploying ships costs supplies. If every ship was forced into deployment first thing, you'd be paying the full fleet cost every battle. A single enemy kite would trash your CR.
It might be neat to combine combat and pursuit into a single larger battle, but one of the reasons they end up as separate fights is to protect FPS performance. Fleet pursuits, by their nature, force the entire defensive fleet into battle. If you were to retreat with a full fleet, you'd have a gigantic lag ball of ships on the field. The game doesn't like that. It instead forces you to have a normal battle, where limited deployment points keep you from having all your ships active at once. You can either deal enough damage to score a free disengage (so the game doesn't have to place your full, laggy fleet into a pursuit), or die badly enough that a non-laggy pursuit battle becomes possible.