Because they are not going to repeat themselves and say If whales were fish, they would have gills, but they have lungs, which means they are not fish. It's simply too complicated- the original is simple and clear.
First of all, @emsr2d2 didn't say it was conditional. She said you could think of it like a conditional. Anyhow I can't see why it would make the slightest difference either way. And you should probably look up the term hypothetical.