Why is "antithesis" not prounced "anti-thesis"

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When the anti- prefix is attached, the stress pattern of the word changes. The stressed 'the' syllable in 'thesis' is no longer stressed when combined with 'anti-' so it undergoes vowel reduction. In English, vowels that are not stressed become reduced to 'UH' or 'IH' (depending upon what dialect you speak). This is, I believe, what causes the main difference in pronunciation between antithesis and anti-thesis.