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
a giant Reaper being constructed, and it looks like an enormous human body:
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?