You are told to check the wheels for any damage, cracks, rust, metal shavings, loose or missing nuts or bolts, excessive grease, or loose lug nuts. That’s at least seven distinctly-different items needing attention. You can walk by a school bus and look at one wheel/rim in just a few seconds and say you looked at it, thereby legally being able to check off that box on the list.
But did you REALLY look? The wheel looked fine yesterday, and the day before, and the day before that. What could have changed since you parked the bus last night?
While you were at it, did you notice that slight bulge in the inside wall of the driver-side front tire? Or did you look, but without a good flashlight? Yeah, you probably looked and all you saw was a black tire shaded under a bus and early morning darkness. But heck, at least you can check off that list, right?
What happens, then, when the tire explodes while doing 55 on the interstate? What happens when the wheel falls off while your driving down a mountain road? The checked-off list did little good.