Why do the majority of construction site workers wear their hard hats backwards?

The back brim is wider so if you wear it backwards, it gives more light shielding for the eyes.

In a lot of cases it’s because it’s easier to look up, especially when a load is being hoisted by crane, without having to look under the visor (even as short as it is).

In other cases, workers who have to wear flip-up face shields when working with grinders find it a better fit to attach the shield so that it doesn’t have to clear the visor when raising and lowering it. (Plus there’s a smaller opening at the top of the face shield that way.)

In addition, if the entire hat is turned around (meaning the suspension as well as the shell itself), then a lot of modern hats that have an adjustable fit have the knob to adjust the headband tightness right at the forehead instead of the back of the neck, where it can be harder to reach.

And some just do that because they’re told not to.

In addition, I would not expect that it’s “a majority” who do so. Perhaps a majority of those you have seen, or a majority of those on several sites that you’ve visited, but I highly doubt “a majority”. For one thing, workers in Asia – where hard hats are worn at all, anyway – are generally required to be very uniform in appearance, right down to wearing hard hats “to the front”, and including chin straps, as well.

Because they’re gittin down wiv their build a homies?

Of course it depends on what style of brim there is on the hat. A full 360 brim like I previously mentioned does what I said and keeps the rain from going down your neck when it rains. The hats with only the front brim could be turned backwards for better overhead seeing, like @CWOTUS said, but turned around will make safety goggles* easier to take on and off, storing them above the rim of the hat. (* Or welding helmet, full face shield, gas welding goggles, general safety goggles, etc. Debris on the work site could come in all forms and from all directions.) And helping to keep the rain from going down your neck.

Increases their forward visibility and reduced painfully burned necks.

Your eyes often automatically avoid hazards. By the time your neck hurts, it’s fried.
Same with rain. Off your face and down your back or down your face and off your back?

@kritiper is correct. I used to wear a hard hat with a 360 degree brim, and the back is slightly more pronounced to deflect things, protecting your neck. My company would not allow employees to wear them backwards, or even tilted to the side.