We also have fantasy land with potatoes, tomatoes and so on. Besides it's not like halflings never trade. Just that they don't rely on trade to live ... at least according to MToF. Personally I view that book as one option specifically for FR.
I think the point remains - if you don't trade much, you don't have cosmoplitan cuisine. You have regional cuisine.
That's why we have characteristic foods of New England, the American South, Southwest, California, Midwest, and Northwest, you know. Because you don't actually get cosmopolitan about foods with just a little trade. You need
a lot of trade before you food cultures begin to mix.
And that isn't limited to the US - the cuisines of Northern and Southern Italy are notably different - and it isn't like they've lacked trade any time in the past couple of millennia.
As another example... before 1900, and the blight, do you realize that grains were not the staple starch for much of the United States? Grains grow really well in the plains, of course, but in mountainous areas along the Eastern and Western coasts, chestnuts were the king of starches in many people's diets, especially if you were in lower economic classes. After chestnut blight killed off most of the supply, trade in grain rose considerably.
So, if you live on the East or West coast of the US, historically speaking you owe your bread... to trade.