The incentive to fortify is already low. It costs a lot of time and pushes you back. That investment already puts you at risk of being outplayed. 


On top of that the military defense system improvements also cost upkeep. 


It is like turrets in RTS games: Only the newbies build them, as they cannot move and cost energy upkeep, and can easily be avoided. 


It is the same problem with system improvements that are geared towards defending against invasion.