In all the Cosmere books, on all their worlds, the only thing <brazenly> different about the ecosystems than Earth are the things affected by magic. So, in one of the books, you mentioned, like, a dolphin. Why is everything the same?
So, there's an in-world answer and an out-of-world answer. The out-of-world answer is that every book has something we call a learning curve, so that is what makes it harder to get into the book. And learning curves, usually, if they're written well, have great payoffs, but also act like a brick wall when getting into your book. And so certain books, I have designed to have shallower learning curves, so you can focus on learning all these new characters, and the situations, and the political situation, but not have to learn entirely new terminology for the setting.
In-world, most of these worlds were created by people or... who came from an original world. For instance, there was a planet that the gods from Scadrial (the planet of Mistborn), they came from this world, and they created what they had seen there. So the animal life there, they just created in their own world, because that was what they had a pattern for.
So in-world, there's a reason, they're all connected. But you'll notice that some of the ones with the steeper learning curve are the ones that have the weirder world, and Way of Kings has the steepest.
On Roshar, the only area that isn't affected by the Highstorms, Shinovar, is basically like Earth, where everything else has changed.
That gives us a little bit of a frame of reference to contrast things with. There's some in-world things about that you'll learn eventually, but it gets us some ways for you to be like, "Oh. The grass looks like this because it is so different from the grass that this girl is pointing out. It's grass like ours." It's very helpful for establishing a conversation topic.