Here you go. I quoted it so you could read it. It’s also Imperfect so I’ll explain again. Take, a jaw. In mammals the zygote becomes a blob, elongates, gets some foldy stuff that looks like gill, gets a mouth, and then an anus. The gill-like things become the jaw bones, and three ear bones of the animal. The ear bones are involved because ancient tetrapods heard with their jaws made of four bones each, but mammals evolved specialized ears out of the bones associated with the joints.
In contrast an ant just grows some spikes off their skin to bust out of the egg and after being born develop little leg things, some of which fold into mouthparts. Also in ants the hole that develops first is the anus, and the body plan is subtly flipped upside down.
Imagine both of those happening at once. On one half would be tons of ant legs , lots of them trying to fold into various body parts but other stuff getting in the way, and on the other there would be a bunch of foldy structures trying to become jaws and ears but getting immobilized by the proto-exoskeleton and opposite that on the same side the mammalian tailbone would be preventing the ant genes from folding their legs into a freaking mouth on the creatures butt. Remember: ants treat hole a like an anus while mammals treat it like a mouth. Same with hole b but reversed. No digestive system is going to survive developing in ten way and four directions at once.