Is it really inconceivable that someone who has literally driven tractor trailers more than a million miles has seen dozens of people killed in accidents?

No, not at all. I'm just a regular middle aged driver with perhaps 300k miles under my belt and I've passed numerous fatal accidents on the roadway.

I just sat back and tried to estimate (since of course, I don't have a CB in my car and can't follow traffic details like a trucker) but I'd guess I've driven past at least 5 fatalities -- and that's just fatalities I've noticed (body on stretcher, read about it on the news later, etc)

Many car drivers forget a basic concept of physics named inertia, which is the reason why heavy vehicles (ex. trucks) take ___too long___ to brake.

Emphasis in too long, because this is the reason why road signs announce dangerous sections of a road with even miles in advance.

Whenever I'm hauling >10K lbs with my pickup truck, I get most nervous going downhill through mountains. The only thing between me and hitting something are a thin set of brake pads on all four disc brakes and the extremely limited compression braking on a ~6L engine.

40,000 people a year in the US. I wouldn't be surprised at all if an experienced truck driver has seen dozens of dead people.