This is directly caused by UPC codes being numerical and short, while 2D barcode have significantly higher, and often ASCII-space, data density in which human readability does not bring much of an advantage.

What does 1D barcode mean? I can think of no bar that can be represented in 1D.

A normal UPC barcode, as the parent said. The data is read along the horizontal dimension. The only reason the lines extend in the vertical direction is to make them easier to scan.

You're taking it too literally. 1D is the industry term for such code, to mean that data is encoded in one direction/dimension.

Getting hung up on the fact that the printed code needs some height to work isn't productive to the conversation.

i'm not hung up on it nor trying to be unproductive. i asked a simple question and then get chastised for it. that's not productive at all. In fact, someone much posted a much more productive response much earlier than you did, so you very much added nothing to the conversation