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Because it doesn't need one.
It's not definite (you're not talking about some particular bread that had been mentioned before) and it's not a count noun in this sentence.
Mass nouns don't have an indefinite article in German, nor in English unless you count "some" as an article.
Not in general.
"ist" usually has no word stress in a sentence ("Er ist Brot" would be "Er ist BROT" with one word stress), and "isst" usually has a bit of word stress since it's more of a "content word" ("Er isst Brot" would be "Er ISST BROT" with two word stresses), but sometimes you want to stress another word for emphasis and then even that difference gets lost -- for example, "ER is(s)t Brot" for "It's he who is/eats bread" would sound identical.
If you ignore stress, the words sound completely identical: a "short i" sound, a voiceless "s" sound, and a "t".