A pickup truck is a very impractical vehicle for a family. And if you aren't hauling bales of hay or the like regularly, it is kind of impractical for any other purpose. Yet pickup trucks are hugely popular, especially with young men.
I think the main appeal of pickup trucks to most of their owners is as a symbolic display of masculinity.
Displays of masculinity should, I would think, need to appeal to women to be useful in mate selection. So why would women find a man with a pickup truck attractive?
The sociobiological study of mate selection has wrestled with a signature puzzle: the peacock's tail. The giant, bright tail on the peacock would seem a huge hazard to peacocks - attracting predators and slowing down the peacock's escape from predators. The ingenious answer to this puzzle that sociobiologists have come up with is the "handicap theory." The peahen, who tends to select the peacock with the biggest and brightest tail, may be thinking that if this peacock can survive despite the gigantic handicap on its butt, it must have tremendous genes.
I think the pickup truck bed is something like the peacock's tail. The pickup driver's mate may reason (subconsciously) that if this guy can succeed in having enough resources to afford a vehicle, despite the huge handicap it imposes on having friends or family, then he must really have something on the ball.