They are not just machines, though, they're more a hybrid of organics and machine. They've been harvesting organic life for millions of years like the Borg from Star Trek (the first harvested race probably the one that created them in the first place). With the knowledge of all these highly advanced civilizations they wiped out (compared to which humanity are just a bunch of hicks in comparison) one'd expect them to be have several contingency plans available if plan A, B or C backfires.
For instance, since the Reapers originally built the mass relays, wouldn't they be able to shut them all down if necessary, winning immediately? Yet they attack Earth directly and come off as simply attacking Earth because they have a grudge against Shepard.