Comment by LocalH

Untrue.

Let's suppose we fill odd fields with black, and even fields with white.

The end result when played back on a CRT will be the entire screen flickering between black and white with a lot of flicker, phosphor persistence notwithstanding.

If you simply average the even and odd fields, you will be left with the completely incorrect result of 50% gray and zero change between image frames.

You seem to be under the misconception that two consecutive fields contain image from the same moment in time. This is not necessarily true with proper interlaced video.