In my experience a significant part of Amazon's inventory isn't something they'll send outside of the "domain country", e.g. trying to send from .de or .uk (this was before Brexit) to .nl.
It just comes down to suppliers, who aren't serving customers outside of select markets for whatever reason.
I see that as a "market platform" problem. I used extensively amazon.de/co.uk with deliveries in Romania, in early 2010's for a bunch of things. But since then they also opened up their market to any seller, quality dropped, shipment became preferential, or 1 cart could result in 2 separate shipments.
Aside. I've seen the exact same problems with our local Amazon "competitor". As soon as they became a platform marketplace I've started using them less and less because of the same quality/delivery issues.
It's not really a problem of the platform, except insofar as you'd like Amazon to only ship from their own warehouses. It's just Amazon reflecting reality on the ground in the EU.
Which is that even if it's legally a single market it's common that stores that deliver something to your door only sell their products in their own native country, or only within their local region.
It just comes down to suppliers, who aren't serving customers outside of select markets for whatever reason.