The general answer to the OPs question and many more like it is either:
1) They are not paying ANY attention to the actual task of driving
or
2) They are not looking any further down the road than the bonnet of their car.
I actually followed someone the other day who overtook a truck, which was doing about 35mph, and then immediately moved back to the left hand lane, even though there was, clear as day (in fact, even clearer than that) a large tractor, complete with bright orange flashing lights doing about 15mph in that lane about 50 yards ahead of the truck! At the last minute they did a full emergency stop and avoided ramming into the back of the tractor by about 3 inches.
Tractor driver had his head in his hands, shaking it side to side in disbelief.
(tbh, as the tractor was massive, weighed probably 10 tonnes, and the car was a nissan note, i suspect any impact wouldn’t even have scratched the paint on the back of the tractor…….. 😆 )