There are four big differences between a GC2 barebones "orbital ship" and the defenses in SEIV (mines, satellites, fighters, weapon platforms) or MOOII (planetary shields, missile silos, fighters, orbital starbases).
Specifically:
1a) In GC2, you get one ship.
1b) In the other games, you get as many ships as you can build, plus all the static defenses, at once.
2a) In GC2, you pay upkeep on defenses.
2b) In other games, static defenses require no upkeep.
3a) In GC2, planets require facilities to build starships to build defenses.
3b) In other games, no starship-construction facilities are required to build static defenses.
4a) In GC2, defense is just as expensive as offense.
4b) In other games, static defenses are way cheaper than offense.
GC2 is all about offense. For much of the game, there is no concept of discrete defenses (that cannot be used offensively). Military starbases are just as good offensively as defensively, for example.