When I lived in Japan, I had access to very high end whisky and sake that was not available in the US. When I visited the US, I wanted to share some of this with my friends and family.
One good bottle of sake is 1.75 liters. One good bottle of whiskey is 0.75 liters. It doesn’t take many bottles to surpass the limit, and the items were definitely unique enough to warrant bringing.
If you start talking about things like Christmas presents, it starts getting nuts. All of your friends want you to being them the good stuff that they don’t have allocations for in the US.
Certainly if you're driving around in a camper.
It just happens to be the case that I don't drink or smoke. There are other restrictions like meat, plants, blah blah, the list goes on.
This isn’t necessarily true. Many great whiskeys and spirits aren’t necessarily available to purchase in other countries. Another commenter mentioned the case of Japan. Many great whiskeys and sake produced in Japan aren’t available to buy in other countries.
Apart from that there is a matter of taxes.
Norway, for example, allows only 2L of spirits or 200 cigarettes IIRC. If you're a heavy smoker that's less than a week. I believe Switzerland is similar.
I don't smoke or drink, but I'd imagine that would be fairly easy to get tripped up by if you were travelling around Europe.