It's true for many countries. The biggest reason is because they are not in the EU. The logistics of getting stuck at some border control is something most sellers don't want to deal with.
Last time I checked the Netherlands we're in the EU and there was no Nexit.
Switzerland is different, though. They are indeed outside the customs union. (Which is always interesting to watch, when travelling across the border in trains and passengers are confused by the radom customs control, since they are part of Schengen, where people, not goods, can travel freely)
Well, I don't understand what's there to check, I never said Netherlands wasn't in EU.
The most probable reason why you can't get packages from EU Amazon to Switzerland is because they are not in EU.
The same reason applies to why you can't get packages to EU if you're trying to get them from US.
Yes, some big sellers don't mind but small ones do, and EU Amazon does have a huge variety of sellers.
For example, I've lived in Croatia before and after EU membership. The Amazon situation changed completely and now I can get packages from any EU Amazon.
I live in Finland (which is in EU) and Amazon (or their European sellers) is the only major web store inside EU that does not ship most of the products here. German Amazon even regularly pushes ads for products that can not be shipped here.
In contrast, most non-EU stores (except Amazon again) happily ship here, even if the shipping fees may be extremely high and border control is left to buyer's problem.
