It is because there is no 'upwind air' inside the middle part of
the jeepney while it is moving. It is just more or less 'stationary
air', hence the fly does not require more effort to keep up with
the jeepney's speed. The wind only blows up to the inner sides of
the jeepney.. Now if the fly were on that area, it would really
need to flap its wings like a supahfly to keep up with the
jeepney's speed.