Many things in nature are sub optimal, imperfect solutions that work well enough. An advantage needs to be significant enough that it creates an advantage in reproduction otherwise it serves no purpose at all. Evolution is a competition of generations, not individuals.
DI implies perfectly designed and specialized creations, something we don't see in nature at all.
Why do golden retrieves have bad hips? (For that matter why do human's have bad backs?)
Why do male black widow spiders hang around to be eaten after reproduction?
The answer is that bad hips and backs don't affect reproduction much at all and being eaten after reproduction doesn't prevent reproduction in spiders.
DI doesn't work as an explanation because it doesn't offer a useful model for predicting or explaining the variation of species.
Evolution is believed in science because it is the only explanation that fits all the natural evidence we've discovered.