Because we've known damn well for decades that it's an incomplete theory! If it cannot be unified with the other best-supported theory in the same domain (General Relativity), then it is incomplete. Period.

We don't actually know that. Weinberg has suggested that GR might be asymptotically safe. Most people seem to think this isn't the case, but no one has been able to show that he is wrong. We can rephrase your argument, and instead of putting weight on theories for which we have no evidence, dump the "built up" probability mass on the idea that the two theories don't actually disagree.

Certainly the amount of "contradiction" between GR and quantum field theories are often overblown. You can, for instance, treat GR as an effective field theory and compute quantum corrections to various things. They are just too small to matter/measure.