And the Council said the Reapers were a Geth construct and the Dalatrass says curing the Genophage was a dumb idea, characters aren't omniscient and they aren't always right; I'm sure the intend was to tell the player that conventional victory was out of the question, but it can be read as something else. My point is that the game has all these little instances where the galaxy as individuals can at least hit the Reapers and that the driving focus for half the game is gathering forces to conventionally take back Earth (which doesn't make sense as a stalling tactic). Plus, his insistence that the Reapers can't be defeated conventionally seems a little daft when his plan seems to do exactly that.
Again I'm not advocating this is the way it should have gone, but I think the game very easily creates this expectation. If it was the way the game ended I wouldn't have questioned it anymore than the Crucible's way of solving it.
I don't remember when do we exactly gather forces to take Earth back. First mission is to get on Palaven to get Turian Fleet, Turians say they need Krogan, Krogan say they need Cure, we go to Sur'Kesh for cure, then to Tuchanka. Turians now have Krogan, we have Turian fleet. Then we go to Citadel and then to Rannoch to get Quarian or Geth fleet. Then, it's about finding out what is Catalyst.
Then, we realize that Ground War is required (because of beam) and Hackett comes up with the plan and assembles "Hammer", that's basically it.
As to "why do we even need fleets then" - Crucible is space-flying thing, it's also very big and would require a hell lot of defense force, even with all it's armor.