Evolution is utterly absurd and is based on nothing other than a similarity between different species.
Therefore they conclude that one must have derived from the other because, well, why? ... because even they ironically (though implicitly) admit that the odds of these species developing at all is so infinitesimally small that they can't even convince themselves that more than one could have evolved independently from another. Therefore, one must have come from the other. That's the underlying assumption which ironically undercuts theri own rationale for the entire thing in the first place.
Well, the similarity in design could just mean, duh, that they had the same Designer. But that's their other premise ... there is no God, and no Designer.
So if you combine the two unproven (and unprovable) assumptions that there is no Designer and the chances are basically zero ... that's why they are able to confused peole's brains into accepting it (without of course explicitly articulating these premises).
It's like this. If I walk around on a beach and find a wristwatch. I'd pick it up and realize. Well, this was designed. It was made to tell time, etc. It did not just evolve here over millions of years. Odds are basically zero. But they tried to hide this from people when duping them regarding evolution.
Now I walk around and I find another wrist watch. This one is very similar (though not exatly the same). But it's also clearly made to tell time. Inead of saying that, yeah, this watch too was designed (by someone), they say, well, this watch must have evolved from that first one we found because ... what are the odds that two fo them would have evolved independently in such close proximity. Well, the odds are the same as if even one of them could have evolved independently. Zero. But they confused you with their nonsense.
Relatively recently, geneticists have found that with genetic mutations and random changes, information is always LOST, not gained. So it's not possible that a mutation would bring about some new feature, but only the corruption of an existing feature. That was the death knell for evolution, and in fact many geneticists (even non-believers) have rejected at least Darwinian evolution.