The design is so that you have plenty of time to complete all objectives, and also bonus missions. With low gear and new to the mission, that time is necessary. With higher gear and more experience it is easy to complete many of the mission objectives with lots of time remaining. Some mission objectives can be completed before bonus objectives completed, and I would hate to be kicked out of a mission when I still have a satellite or a bridge or a comm tower to destroy.
So maybe they could implement an exit option, where if you are done with the mission objective, you get the credit. optional. Seems easy to do. Ive often wondered the same thing.