Either they are too drunk or they are on their phones while they are driving talking or texting to someone else or they just in general lose control of the vehicle while driving and end up crashing into someone else or swirving off the road
Falling asleep at the wheel is a major factor. 1 out of 25 drivers report having fallen asleep at the wheel in the last 30 days, resulting in more than 70,000 crashes and more than 40,000 injuries and 6,000 deaths per year.
But a big one is not having good shocks so the vehicle rebounds when changing cornering directions and usually when they panic after getting to a slide by letting go of the gas or hitting the brakes so that they lose all traction or gain it back too quickly.
Speed going too fast and things get out of control. Speed would be the cause of most accidents on the Highways. Its not actually the speed that kills you its the sudden stop
just because it can doesn't mean you should.