It is just convention. English spelling is not 100% consistent or regular. There have been many attempts at spelling reform, but none so far has gained any traction.
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ecossaisexcept add, ebb, innThese are one-syllable content words that begin with a vowel. I think that, psychologically perhaps, we like content words (nouns, verbs, etc., rather than determiners, prepositions, and conjunctions) to have at least three letters.