Earlier quoted context omitted.
Despite using the same platform, no two companies setup their product catalogs the same way, from how multiple products/colorways are grouped together to metafields are used to communicate more nuanced product features. There's simply no way Shopify could hope to present a unified catalog given those implementation nuances.
Yep, this is pretty much it. Amazon has the advantage that they are able to force their sellers to specify metadata about their products in very specific ways, which allows them to build on top of that consistency. With Shopify, as tomnipotent said, its all over the place, and it only gets worse as you cross languages.
Probably hands-down the most irritating metadata issue with Amazon is when sellers don't include quantity information properly - or even include quantity in the title when selling just one size - which I'm sure they do on purpose to make price per unit shopping more difficult.
Sometimes the way the items are listed will be different between one product and another made by the same company, or between Amazon's listing and a third party's listing for the same general product...and they each have different types of the item available. Say, Amazon will sell Ken's Socks in purple and blue, all sizes in purple but only small in blue...while Blankenship E
Things seem to get especially messy when you have a matrix of two 'flavors' (such as a sub-model and color, or unit size), and not all combinations were manufactured or available.
Even the UI isn't consistent. Sometimes it's a drop-down list, sometimes it's the rectangles.
Oh, and often the model/part number isn't entered...