If they're so awesome, why can't the reaper on Tuchanka fire its beam horizontally at Shepard killing him/her? Why can't the reaper on Rannoch fire its beam at Shepard horzontally killing him/her? Why can't the reaper in London fire its beam at the missiles on the vehicle? Why doesn't Harbinger fire its beam at the Normandy in front of the beam? Why can't the reapers fire their beams horizontally at the approaching fleets over Earth?
It literally boils down to rule of cool, Bioware wanted all these cool cutscenes and sequences, all at the expense of consistency, common sense and a coherent plot, something which I really hope they don't do again in future installments. I really wish Bioware had resisted the urge to make Mass Effect 3, and to a lesser extent Mass Effect 2, blockbuster action movies, and given us a consistent and well written story instead which stayed true to established lore.
Yes, this would have meant that the trilogy would have ended with the Reapers steam rolling the galaxy and continuing the cycles forever, but hey, no one could turn around and say that ending wasn't consistent with what we knew about them or, oh god I hate this phrase, was full of "plot holes". If written well, the journey towards inevitable defeat could have turned out to be one of the most engaging, emotional and impactful stories in the history of the medium, but nope, we got a summer action movie and rule of cool instead, with all the bad writing included.
Such lost potential.
But, oh yeah, Genophage, Krogan dumb, Salarian intelligent, so sabotage it!!!