We're just suspicious people, man. Kept me from ever being raped and dumped in a ditch. Let's say there's a 99% chance you're awesome and 1% chance you're a murdering/raping/stealing shrimp kind of dick. So if we assume you're awesome, 99 times out of a 100, that's okay, but that 100th time could really fuck us over.
Fancy speak would say something like "The amount of risk determines at what odds being wrong is acceptable."