Because mother nature has perfected the fish design so well over the last 350 million years. Flounders and Sole. The skull is twisted with both eyes on the same side of the head, usually, they have no spines. Next time you see one, try to work out if it is a left handed or right handed Flounder. Try to work out how Sole differ to Flounder.
Hint Look for dorsal fin and the gill cover.
The gill cover is more visible in Flounder. Their body is covered with plates or spiny scales and their gill opening is a mere sideways slit. The teeth are either fused or with six or eight large front canines. The first dorsal spine is lockable in Leatherjackets. Boxfishes, Puffers, Triggerfishes, Porcupine fishes.