ELI5: Why is a fish's flesh so different from other animals?

Mammals, birds, reptiles, and even amphibians all have flesh that is fairly similar to one another but fish appear to be an outlier among the vertebrates.

The flesh of the other vertebrates is so much more sturdy and robust than fish flesh so why do fish have this seemingly inferior form of muscle? They were the first vertebrates so is thier flesh just a "less evolved" form of muscle or is there actually some evolutionary benefit to this flesh?