"Fixed fortifications are monuments to man's stupidity"
-Gen. George S. Patton
In modern warfare, I tend to agree with Patton's assessment. The sheer power of explosive charges that can be directed at a target mean walls can easily be breached, and safely by artillery, armor, and air. In a battlefield environment where speed is of the essence, a fixed fortification simply makes you an easier target.
This is a far cry from the fortifications of Vauban, brilliantly designed to offer cannons the ability to direct fire in nearly any direction and to effectively protect troops inside.
I understand that forts offer a compelling strategic gameplay mechanic and a way to securely hold key territory. I think Firaxis will include them for this reason, but as for realistically viable sci-fi, as soon as air power and satellites come into play, fixed fortifications fall into Patton's territory.