Yeah that's a good question, and bu

Yeah that's a good question, and bugged me a bit when watching the film.

I figured it was at least partly due to the rationing. If you recall, shortly after he leaves his aunts house he is told that everything is now rationed, presumably you are given your share and can't buy any more. Also remember when he tries to buy rice from the old farmer that he is eventually refused, as the farmer claims he doesn't ave enough even for himself.