That might be an issue indeed, since pickup trucks were typically designed to be relatively cheap and utilitarian. My 1970 Cougar has pretty curvy window glass and no wing windows, but if it were just two years older, it would have had wing windows and much flatter window glass.
But I’m inclined to think that the cost of the wing window itself, with its frame and hinge and latch, offsets any savings by having flat window glass.
Perhaps you’re right, but I’m not so sure. I’m no expert and I’m really just thinking out my ass, but I can’t think of any cars or trucks of that era, regardless of whether they had wing windows or not, that used large, curved glass pieces for side windows. That has to be a technological engineering issue. There were certainly cars that were curvy and could have benefited from such windows.
It’s the polymers used in safety glass, which is just two layers standard silicon glass with a plastic sheet between them. As plastics improved, so did safety glass, eventually allowing for stronger, larger, and more curved glass sections.
It was common for a while to have a choice of a body style with or without vent windows. My 1970 Cutlass S, which came with factory air, did not have them (or B pillars) in a body style called a Holiday Coupe. The Sport Coupe did have vent windows. The best vent windows I’ve ever seen, are the ones on my friends 1930 Hudson. There are two cranks, one crank cranks the vent window open (and keeps it open at speed not like the floppy manual ones) and the other crank cranks the whole thing down into the door for a clean look (it’s a 2 door convertible).
My '55 Speedster had plastic and canvas side-curtains on frames that inserted into plugs in the door. Glass was heavy. You could only use them when the top was down. With them in the top would not lift like a sail and pivot on its tiny central mount.
It also had no gas gauge, no radio, nothing that’d add weight.
I’m thinking of my mother-in-law’s 1965 Mustang convertible, which had wing windows and very flat side windows, compared to the door glass in my 1970, which is much curvier. Maybe not as curved as, say, the wraparound windshield in a '56 F100… but anyway, that windshield would be laminated if it’s safety glass at all. The side windows would be tempered rather than laminated.
The curve in my Cougar’s windows is a vertical convexity that more or less follows the shape of the door. Not an extreme curve by any means, but definitely not flat.
Windshields are much, much more angled now than 20 years ago, it would be a design challenge to fit vent windows on those. either they would look weird and cut-off, or they would have to be huge.
They’re still out there. Besides the obvious musclecars (Mustang, Camaro, Challenger), you can still get Honda Accords and Civics as 2-door coupes. The Toyota Yaris is available as a 3-door hatchback, if that counts. And on the higher end you still get Mercedes and BMW making coupes in several lines.