Why are flat fish flat?

Answer 1

Better question: why are there evolutionary niches allowing flat fish to thrive? Three answers, although you shouldn't consider me an authority:

1. Flounders lay flat on the bottom, and flatness allows them to be less conspicuous.

2. Flatness minimizes shadows on the ocean floor, thus aiding in stealth.

3. In coral reefs, a flat body and a long nose allows fish to get at food inside coral heads.

In each of these three scenarios, it's not hard to understand how flatter fish would be more successful -- and therefore more likely to reproduce.