First off, 600kph is not all that fast. The human eye is perfectly capable of tracking objects at such speed. If you're imagining things just being a big blur as you pass, you're stupid.
Likely your only real world example is a car, and only experience this with objects closer than 10m or so. You're likely flying your plane at close to 150-200m, or more than 10 times the distance, while your speed is likely not even 6 times as great.
Second, unless you're flying perpendicular to the vehicle's direction of view, your speed is fairly irrelevant to shooting you down. You fly straight down their sights, they don't have to compensate for your speed, they have to compensate for your rate of descent, and that's about it.