In short, ignorance.

1. Say bible is perfect.
2. Then why God doesn’t preserve it?
3. It follows that God preserved the bible. This is actual christian belief

But does he? So many verses in the bible are dubious. It means that even the best scholars do not know whether such verses should exist there in the first place for all of us to know.

In other word, God doesn’t, at least not perfectly, preserve the bible. Some verses are added, some verses are removed and no body knows whether it should have been there in the original or not.