Interestingly, it doesn’t look like IKEA uses UPC barcodes at all and just has their own format and numbers. I guess since they only sell their products in their own stores, there is no need for it to be globally unique.

I'm curious why you say it's not a limited resource.

Although you could theoretically expand the length of UPC every couple years to keep growing, the reality is that all the systems communicating these numbers back and forth need to have a standard.

In addition, printed barcodes need to fit within the area they are designed to fit in, if they were regularly increasing in size it would impact various systems (e.g. conveyor systems scanners, item labels, etc.)

One of the issues is that barcode readers were very large and expensive to construct in the mid-century. The railroad application became practical earlier because of the small number of readers and large amount of space available at railyards. Kartrak readers were small-refrigerator-sized cabinets with arc lamps and an AC motor driving a rotating mirror. They required regular mechanical maintenance. The actual logic was done in a minicomputer installed in a nearby building. Between the optical cabinet and the minicomputer, it probably came out to something like 15 square feet for each reader (and kW power consumption for the arc lamp).

Practical retail barcodes had to wait for a lot more miniaturization of the mechanics and pretty much for lasers, and weren't seen until after Kartrak.

You can tell that Kartrak had a rather distinct lineage from retail barcodes - GTE, who designed Kartrak, don't seem to have been aware of the earlier work at IBM and designed their system independently. WABCO developed a competing system that didn't gain adoption but actually was based on the IBM work and resembles modern barcodes much more. The result is that Kartrak is an exceptionally weird symbology, with a number of design traits that were either not seen at all in other barcodes (the unusual half-toned bars for better performance with arc lamp readers) or not seen until decades later (the use of color and offset start/end points of bars to avoid partial reads).