I'm guessing that the real answer here is "because the writers changed their minds," but is there an in-universe reason for why the proto-Reaper at the end of Mass Effect 2 actually LOOKED human? Mass Effect 3, especially Leviathan, clearly established that
Reapers were made from the harvested races but that they were all based on the crustacean-like design of Harbinger, which in turn resembled the Leviathans.
There are a few size variations from this norm, but otherwise all the Reapers look fundamentally the same. At the end of Mass Effect 2, however, you find
So why did the Reaper at the end of ME2 look like this? Did I miss some explanation of that, or did the writers just decide that having each Reaper have a totally unique design wouldn't be worth it? And if it was a change of design, why did the Dark Space Reaper fleet in the final shot of ME2 already feature the consistently Sovereign-esque design, instead of a race-by-race design for each ship?