The Big Bad is sending our hero (and his Ragtag Bunch of Misfits companions) through the wringer. He clearly has the power to wipe them out any time he feels like it. But he's content (for now) with sending out wave after wave of progressively stronger Mooks, Elite Mooks, and the occasional Quirky Miniboss Squad to test our hero. Maybe he's even fought them personally, but let them live, maybe even let them think they beat him. Maybe he even leaves them alone to train for their inevitable confrontation. So why don't they finish the protagonist off and be done with it?