This has created or at least helped to create the stupid action movie standard
Hint: they're action movies. It's not quite the same having the car/person get out in plenty of time and then having the explosion not quite reach when it finally goes off. Slavish adherence to physical principles and consistent scales don't make a good movie.
There is however nothing to prevent an attempt to fire the weapon on Endor before the reactor gets hit.
Except for the complete lack of explanatory dialogue - plus I think there's enough peril already.
DE