I'm sure they Reapers could actuallly travel back....but that would take alot of time. I don't get the human Reaper it doesn't make sense for machines with a God complex to wish to be born in the form of both humans and macines or what makes humans so special.
I got a little theory on this, the Reapers are OLD as we know and I don't care how advance they are...they have to feel the effects of that age. I wonder if at the end of every cycle or certain cycles they need to create themselves literally new bodies or ships to us. There essential machines but they must have some sort of biological parts too them and maybe they just pick the strongest species to them or the one easiest to create there next generatinon of bodies/ships.
It might explain why Earth is one of the first targets.