Oh but from a biological perspective they are a whole different case. The general rule would be that fish doesn't lay this type of eggs (with specific layers and a hard shell). Amphibians neither. And then, evolutionary speaking, came the amniote egg. With feats that work as an adaptation to earth's dry environments, without water. It's really a 'deal breaker' and as such it's treated by biology books because we split all animals between those that have such eggs and those who don't...