As a side note: the word "corn" is closely related to the word "kernel," but "kernel" is countable. So the following sentences are correct: "Where I live, the most popular kind of corn has a mixture of yellow and white kernels on a single ear." "This field of corn has 500,000 corn plants per hectare, each plant has one or two ears of corn on it, and each ear has 800 kernels of corn on it." "This field is growing the type of corn called 'sweet corn,' and that field is growing 'field corn,' used as animal feed." The word "grain" actually has two different meanings. It can be an uncountable noun just like "corn" or "wheat"--or it can also refer to individual grain seeds or kernels. "Including corn, wheat, and sorghum, the United States produced $80 billion worth of grain last year." OR "The king offered to reward the man who invested the game of chess. The inventor asked the king to put one grain of wheat on the first square of a chessboard, two grains on the second, four grains on the third, eight grains on the fourth, and so on, doubling each time."

To use the definition I gave in my previous reply, it's because we don't say 'corns' (corns are hard patches of skin on your feet, but that's another story). 'Corn' - like most other agricultural products is uncountable - wheat, rice, sugar, coffee, milk, cheese, beef. If you want to quantify corn you can use weights and measures, or you can also say an ear of corn, or a corncob.

The answer chosen as ‘best answer’ did not help at all. It did not answer the question, ‘why is corn an uncountable noun’? I have wondered the same thing. My best guess is that it is considered ‘uncountable’ because you can’t count individual kernels of corn. Of course, you CAN count ears of corn. The designation of ‘uncountable’ must have bern made on the basis of corn kernels, not ears of corn.