I think it's just like that because skatole is only one of many odours in excrement, but it's the one that makes in uncomfortable and makes you want to get away from it. So evolutionarily it's useful to just get away from skatole whenever encountered.

So it's not so much that it smells like poop, just is as revolting in some more general sense. And DMT probably has a lot of bitterness to it(can't remember what it tastes like, honestly, but I remember its cousins AMT and DPT being intensely bitter, though I didn't vape them).

Weirdly, skatole also smells nice in very small quantities, and is apparently used in perfume.

So yeah, odour is a pretty strange sense...