That's part of the reason, but it doesn't explain the use of chrome, rectangular or round headlights instead of molded headlights that integrate into the bodywork, and many other design elements that have remained virtually unchanged since the 1970s. I think a lot of it is just that Americans like the look.
TBH, that's Peterbuilt's design language. My guess is that the parent poster was talking about the 589. That's what I think of when I think about tractor-trailers.
https://www.peterbilt.com/trucks/on-highway/589
If you compare, say, the Peterbilt 579 and 589, you'll find that the former is way more modern looking and aerodynamic and the latter is very classic and old school looking. The 579 is also vastly more fuel efficient. I'm fairly certain that the classic design of the 589 is entirely for aesthetic reasons rather than for part interchangeability.
Even a brand new electric cab over garbage truck looks vintage: https://www.peterbilt.com/trucks/zero-emission/520EV