It's just the distances involved. Amazon UK can get everything from any warehouse in the UK to your local depot for last mile delivery overnight, if not faster.
In the US unless you air freight ($$$) it you cannot rely on getting stock from warehouse A (say in NY) to warehouse K (say near LA) overnight. This then means you have to keep a lot more "duplicate" inventory sitting around various places as you cannot just in time move it, which is also extremely expensive.
I imagine the reports of shipping delays in the US are caused by Amazon intentionally trying to reduce the amount of inventory on hand and/or using less air freight.
This is also happening in Europe now on an EU scale. Often something takes a couple days because it goes from Germany to Spain or something. They don't say that's the reason but you can tell from the stickers on the products. I guess they don't do this anymore for the UK though because of all the customs hassles since Brexit.
I've not noticed a major difference in the UK from before Brexit. Some products would come from elsewhere in Europe but most would be local then too. London alone has several Amazon depots, so lots of things are same day delivery here.
Maybe it's down to competition - for lots of products I'd go elsewhere if they didn't offer at least next day delivery.
In the US unless you air freight ($$$) it you cannot rely on getting stock from warehouse A (say in NY) to warehouse K (say near LA) overnight. This then means you have to keep a lot more "duplicate" inventory sitting around various places as you cannot just in time move it, which is also extremely expensive.
I imagine the reports of shipping delays in the US are caused by Amazon intentionally trying to reduce the amount of inventory on hand and/or using less air freight.