I just wanted to give my guess. As most of us know that usually uncountable objects would need to be placed in countable objects. Like rice in sack, water in bottle etc.
In the old days, bread may not be sold in a slice. It could be sold in a loaf only. Since English has a long history, there could be possiblility of how it is being sold.
I am sure we can count rice one by one and we can count water drop by drop but that do not make them countable objects.
To me it's so simple! Some nouns can be countable in one language and uncountable in another. I just stick to the rules of the language I'm using at the moment. I don't argue. I try to learn, and to sound correct. I don't like it the way it's in English? I say it in Spanish.