In the first game a single Reaper controls a Turian and an Asari to get them to make armies, presumably for the purpose of making harvesting the galaxy easier.
You fuck those plans over by the end of the first game, so that come the second one, the Reapers must take out their Prothean husks (why are they still around?) to have them collect only humans to make a human Reaper, presumably for the purpose of Reapifying human life and making it easier to harvest the galaxy.
You fuck those plans up as well, and then all of the sudden the Reapers are like, oh shit, time to just fucking go there and do this shit ourselves. At which point they begin harvesting life (except for the Quarians, Solarians, Krogans?) but in such underwhelming numbers that Shepard has time to fly the Normandy around the galaxy like it’s the fucking Millenium Falcon doing god knows what.
Until the end of the game, at which point the Reapers take the Citadel to Earth (why Earth?) because they found out from an indoctrinated Martin Sheen that it was the catalyst that would unleash the giant space penis (why didn’t they already know this given that the AI controlling them was hiding out in the catalyst the whole fucking time?) at which point the Reapers are tricked into not protecting the one way to get to the Citadel: a space gate on Earth, in London, constructed for the purpose of doing jack shit on the Citadel for absolutely no apparent reason.
Why would anyone have a problem with this?