The thing is, usually we simply aren't in a position to see the defensive arrangements - that is, we cannot see that they wouldn't be there.
In the two TOS movies where Earth is attacked, the invader is a supercreature that simply shuts down the defenses. Verbal mention is made of their existence, though, so we can't really claim Earth would have been undefended. In "BoBW", the invader is again a supercreature, but this time we get to see some of the carnage. But while we see the dead starships, we never get a chance to see or not see the fixed fortifications. They probably are there, and they probably valiantly fought the Borg, but the episode didnt' show this because the VFX would have been too expensive and the drama could be written to exclude this.
In DS9, Earth is again attacked, and defenses react, and the enemy is defeated, but once again we don't see the action. That's cheap and effective, dramatically speaking.
The only time we actually see that Earth is undefended against a predictable attack is this ENT thing. We don't see it in "Twilight", as the camera doesn't dwell on the events long enough. We only see it at the end of the third season - and while it appears to fit a pattern, it's actually a unique event.
Timo Saloniemi
In the two TOS movies where Earth is attacked, the invader is a supercreature that simply shuts down the defenses. Verbal mention is made of their existence, though, so we can't really claim Earth would have been undefended. In "BoBW", the invader is again a supercreature, but this time we get to see some of the carnage. But while we see the dead starships, we never get a chance to see or not see the fixed fortifications. They probably are there, and they probably valiantly fought the Borg, but the episode didnt' show this because the VFX would have been too expensive and the drama could be written to exclude this.
In DS9, Earth is again attacked, and defenses react, and the enemy is defeated, but once again we don't see the action. That's cheap and effective, dramatically speaking.
The only time we actually see that Earth is undefended against a predictable attack is this ENT thing. We don't see it in "Twilight", as the camera doesn't dwell on the events long enough. We only see it at the end of the third season - and while it appears to fit a pattern, it's actually a unique event.
Timo Saloniemi
You'd think by the 2270s that Starfleet would have enough starships, cruisers, et. al. in Earth orbit and around nearby planets and asteroids to provide an adequate defense ring about the planet.
