It wasn’t completely foolproof. Chrysalis had been quite pleased to learn that when the human didn’t set off half the alarms of Canterlot. Still, it wouldn’t do to lack caution and so they chose to impersonate the only pony the human had regular contact with. They could’ve chosen the griffon that oversaw his businesses, but pony forms were easier for them to maintain.  Getting him to hand over the amulet might be a bit of a problem.