Also some people are terrified of dogs so having one sniff over them would be worse than being groped. Not that the TSA cares about feelings or anything though.
By contrast the police here got German Sheppards to sniff for drugs on suspects on the street. It turns out the police didn't actually buy trained sniffer dogs - they just wanted something big and fierce looking to 'encourage' people to cooperate.
Not that the TSA would consider any such thing.