Walkers are a lot smarter than we think
They're hustling us, pretending to be all mindless. Hey look, that rotting looking person is making an "EEEEeeeee-eeeeeeeeee" noise, not "AAARRRRRGH-RAAAAGGHHHH". They're an imposter - let's get em!
And they're quite modest too. After getting pretty much ripped apart and killed by a horde of walkers, new ones very considerately change into relatively intact clothes so as to not let any zombie kids see any zombie boobs or zombie wangs.