That's because on heavy equipment there's stuff mounted up front (implements, i.e. bucket, scoop, blades, etc.) that gets in the way of the lights, so they have to be mounted up high and aimed down in front of the vehicle. From the driver's seat you can only see a certain field of vision in front of the vehicle anyway, and grille mounted lights light that field of vision fine if they are aimed correctly.
Grille mounted lights leave a black hole behind any terrain feature. Is there another rock behind that rock, or is there a hole? No clue. It's just black. How steep and deep is the ravine? No clue, it's just a ribbon of blackness. I can see the terrain beyond the ravine, but that's not my most immediate problem.
Only lights positioned above the eyeballs of the driver will guarantee that light is visibly reflected from every feature within the driver's view. Lights below the bumper leave a shadow behind every feature.
I agree, there is no point illuminating things that could not be seen anway. I don't need 400 yard illumination, as a geological age will pass by the time my truck arrives there. I do need to see everything that is seeable 50 feet in front of my truck. It is illegal, and dangerous to use roof lights on any open road where higher speeds could be achieved. Their use should generally be restricted to off-highway use, which will of course come with lower speeds. I would make an acception here for long, open stretches of smooth dirt roads typical in the SW.
The problem with roof mounted lights, and why some people think they are poseurish, is because so many people are fitting Hella 4000 pencils beams up there, with a 2 mile range, on a truck that tops out about 60-70mph, and most of the time is doing 20-30mph, at best, when off pavement. I agree, those lights are somewhat pointless. But it is the lamp choice that is the problem, not the location.
Tractor work lights provide excellent illumination to 50 feet in front of the truck. They're also very affordable. Unfortunately, they're just not cool. I find affordably priced driving pattern lights to be a reasonable compromise.
Some people don't like roof lights because of glare on the windshield and hood. This is a trivial problem to solve. A simple shield under the light, or setting the lights back from the forward roof edge will solve this.
Furthermore, excessively bright forward illumination can also be a problem. The lighting typically provided by HID lamps blows out your night vision. Your pupils close. You can see everything that is illuminated very well, but you can't see what is not directly illuminated. Looking to the sides, everything is dark.
Use only as much lighting power as required to get the job done.