They have been, but oceans are massive (to put it lightly) and so it takes a lot to raise them significantly.

I both like and dislike that. Because on the one hand, yeah, you’re not wrong. On the other, it feels a bit like it discourages exploration as a kind of “yeah it just be like that” sort of thing.

So this hits the pitfall many fall into of thinking evolution is a thing you choose. It’s not. Mutations, most of random, happen and the ones that are useful in some way, or at least not actively detrimental, get passed on.

The reality is that science still doesn’t really have good answers when it comes to how a species’ chromosome is divided up. Hell, there’s a lot about our own genetics that we still don’t understand. For example, introns are something of a mystery. We know what they are, we know what happens with them. But why? Still not sure.

So the answer to your question basically comes down to “we’re not entirely sure.”

Not satisfying, I know, but that’s science. The more you know, the more you realize you don’t know.