They only trust themselves, their own observations, their own instincts, their own judgements.
That's why Motan managed to successfully bluff Muses and the others.
Of course, nothing is absolute. If Muses had sent a Needle over to negotiate with Motan today, the latter might already be in the gutter or on the run by now.
Unfortunately for him, Muses came in person and fell into his own sense of caution. And the other mainstream thieves, Flower Snake, Blood Chain, and Leaf Hidden were also repressed. They even tangled with the only thief who looked like a child but was less intelligent than the average person - Needle!
"I'm sorry, sir," said Muses, whose voice had initially cooled down but suddenly became extraordinarily warm. He didn't sound like a thief at all, "Needle has a bad temperament, but since you also know that he is of 'subpar' level, please don't take it to heart."
Motan grunted softly, then pointed lazily at himself, "Link Selda."