These phones are sometimes (always?) preloaded with ton of shady apps. I definitely don't trust my kid to read EULA's and understand dangers of agreeing to random permission pop-ups. Even if I pre-screen all apps before giving the device to my kid,
each system update may install new apps and children will definitely not wait for daddy to come from work to review them.
Price, probably. Children are often quick to break fragile things like modern phones. But honestly, in my experience every parent I know buys their kids the same phone they use in terms of the OS.
Who suggested buying your child an android when you don't have one? When somebody asks why you drink bleach, it doesn't imply that you should drink motor oil.