In the God argument, we can't tell if ANYTHING exists whatsoever. Nothing tangible is there. There's a wide range of options there -- anywhere from God exists and fits a particular image that people have come up to the extreme of God does not exist at all.
With the fetus, we DO know it at some point it becomes a baby... and we consider a baby a person. So the range here is that the "fetus is alive but has not yet become a person" to "the fetus is alive and a person." This is a much narrower range. We know it becomes a person, it's inevitable, we just don't know when. If the God argument was expressed in similar terms, our range would something like "God exists and has goals towards humanity" to "God exists and fits the exact image of a particular religion."
This is why the "fetus might be a person at such-and-such a stage, let's show some prudence" argument does need to be considered... except for the fact that we just can't get more granular with it.
It's the same reasoning that comes up in cases like Terry Schiavo's -- we know she used to live and "look alive" and could engage people like everyone else, now we can't see inside her head and know what's going on, so... let's be careful about what we assume and what we do.
It's very prudent and very appropriate.