Well, it was kind of a rushed job, and they needed an excuse to put those characters on the Normandy to try to make the end make a bit more sense. I just tell myself the Normandy's stealth system keeps Harbinger from detecting it and call it a day.
But yeah, that doesn't explain why they couldn't just airdrop by the beam and call it a day.
Maybe its the same reason why Gandalf couldn't just whistle up some Eagles to get Frodo from Hobbiton to Mount Doom, or why Glinda never told Dorothy that her shoes could take her back to Kansas.