Drivers need to pay attention. There's no fog, rain, wetness, etc.

Correctomundo, but I don't believe that many drivers were not paying attention. The person taking the video had a light on the very first car and also during the whole video. There were enough brake lights from cars approaching the wreckage but that didn't seem to slow down oncoming traffic too much. I'm thinking there must have been something distracting that many drivers before they could notice what was happening up ahead. Billboards, neon signs, other drivers can be a distraction that calls for your attention also and can be distracting no matter if you are a good driver or not. "Expect the unexpected."

Afterthought: The distraction may have even been the cars pulled over on the shoulder of the road too.

The fact that both cars were black was likely a factor.

Anyone who’s driven at night on LA freeways would expect exactly this. Between oncoming semi trucks with their brights on, signs, intermittent street lighting, etc, night vision is non existent. Anything not lit is invisible beyond 30 ft ahead.
Also, out driving your headlights is the norm almost everywhere. Low beams illuminate about 150 ft ahead, at 65mph it takes over 500 ft to stop on average (according to google), farther for heavy vehicles, worn brakes, and trucks. A recipe for disaster.

Not that it seemed to apply to most of the vehicles that crashed but a lot of modern cars with LED or HID lights have a VERY aggressive cutoff, with the driver's side light being angled down slightly to minimize blinding oncoming traffic. Unless you have your high beams on (which you can't really on a busy highway), it's quite easy to outdrive those lights on a poorly illuminated section of highway.

On another note, I wonder what happened to that dude in the red shirt that got back into his car just before it was hit. I didn't see him get out... Also, I wouldn't be milling about in the middle of a freaking highway; I'd stay in the car or maybe even better, get to the damn shoulder as soon as you can!

I didn't see him get out either but I think he climbed over the divider where it was safer. He's in the dark but you can just see his silhouette moving around at 1:05 all the way to the left.

I understand people don't like being good samaritans by default, but it would have been smart (and nice) to just leave your car in front of the cars with your emergency lights on and just wait at the side of the road. Even better if it's a big rig or a large truck. You'd likely prevent more cars piling up for the price of your patience and time. And, depending on how good you are at Frogger, your safety.

The camera guy obviously bought himself an old police car, probably had emergency lights on, drawing attention to himself.

People rubbernecking the "cop car" at the side of the road and missing the dark coloured wrecks.