I might well be wrong, but in the various conversations I've had with the people involved in the field, none have clarified why the BBT isn't using light speed as the denominator. The story is that eventually the universe will grow so large, other galaxies will fade, because their light can no longer reach us. It is the ruler used to measure the expansion.

As Einstein said, "Space is what you measure with a ruler." What is light speed, aka, lightyears, a measure of, if not intergalactic space?

Like I said, try not to think of it in terms of sociological implications, but the basic logic.

As for why we might have an oversized opinion of our knowledge, we are the center of our understanding. As the old saying goes, the more you know, the more you know you don't know.