The design was driven by regulations that limited to total length of the truck but not specifically the trailer. Smaller tractor, bigger trailer, more carrying capacity, more profit. Sure, visibility is shite, but if it is legal, they will get it, because carriers are here to make money.

But it turned out not to be legal, and regulations changed, now specifying different limits for the trailer and the total length, meaning it lost its main reason for existing.

Sure, visibility would be worse than on a normal truck, but no worse than a low sportscar (which is also road legal and no problem to drive). I don't think it would have been much of an issue.

It's a bigger issue than one of legality. The high seat in a standard truck cab offsets two of the biggest challenges with driving a truck: it takes ages to get up to speed, and a very long distance to stop. High visibility gives better sight lines for further so the driver can plan. That helps to improve fuel efficiency by better anticipating traffic and planning acceleration accordingly. It's absolutely vital for safety because the driver needs to brake for what's happening a huge distance up the road. If you don't have that line of sight, they'd have to drive so defensively ("stop in the distance you can see to be clear") that any efficiency gains from the aero would be completely swallowed by all the accelerating and braking.

A sports car of course can stop in a heartbeat and the excess power means it can easily manage its flow in traffic. They'd perhaps not even be legal if they took as much road to stop as a loaded truck. It's four times the distance - 150ft for a decent sports car at 70mph; 600+ for a semi.