The biggest chunk is in the Netherlands, but you want your gold also in other countries, so that you can still access is (and don't loose it) in case of an invasion. Hence almost half is in North America, a completely different (and (somewhat) allied) continent.

It's also easier to sell in certain markets, such as London.

Not just so that you don't lose it in case of an invasion, but it is proof that you would be willing to help out the host country in the case they were invaded. Also, that you would not invade them.

In the olden days, the local lords would marry their children together, to create familial groups and ensure that for the forseeable future the local areas would not have a conflict. Modern democracies don't have such valuable family members, we use valuable objects instead.

Just to be a pedant, 18.5 % in the US and same in Canada so 37%.

I wouldn't call that almost half, it used to be about half before the move (NY at 31.3, 19.7 in Canada).