Your own foolishness right back at you.
First, there were millions upon millions of marsupial animals in Australia, once it separated from the rest of the continents. Yes, there were many species of marsupials, that couldn't breed with other species. And within each species, there were millions of animals. Evolutionary change, when it happens, starts with (usually) very small changes in some -- but not all -- individuals. Small enough that anyone would consider them the same species: a little taller, a slightly different colouring, teeth shaped ever-so slightly differently, a little more elasticity in the bounce, or whatever. But otherwise pretty much indisinguable from the rest. But if those tiny differences offer an advantage, and can be passed on genetically, then those tiny differences will eventually show up in the majority.
This can take generations. And the tiny differences keep happening, and after hundreds or thousands of generation -- especially if groups get separated from one other by some feature of nature, eventually you could have a herd of extremely similar (but not genetically identical) animals that we may call kangaroos. And their parents we would call kangaroos, and their grandparents!
But if you were able to look back much further, say hundreds of generations, you would turn up something that you would be forced to say was "different enough that we ought not call it a kangaroo."
Kangaroos and wallabees have common ancestors. Some species of kangaroos and wallabees can mate -- usually producing sterile offspring, occasionally producing sterile males females, and even less often, fertile females but not males. Yes, kangaroos and wallabees have many features in common, and yet they are different species.
The pixels I showed could be presented by the computer program that generated them one after the other, rather than in a single picture (in fact, that's how the computer actually does it, just so fast that it looks "all at once"). And in that case, my challenge to you, to decide which of those millions of pixels was no longer orange but red, and which was no longer red but violet. And you couldn't do it.
Your failure is to think big enough. Every who does this imagines 2 animals (or if they're really brave maybe as many as 11). But they refuse to do over millions of animals, over hundreds or thousands of generations. And that is why they simply can't understand what is being said to them. It is an utter failure of the imagination, and nothing more.