If Allah is the main god why do non Muslims not believe in him?

Allah is "the Deity" in Arabic language and is worshiped by Muslims. Non-Muslims worship "the Deity" and name Him differently in their own languages.

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Christians aren't non-muslims, they believe in the same God - but call him Jehova/Yahweh so they do believe in Allah.

Jews aren't non-muslims, they believe in the same God - but call him Elohim so they do believe in Allah.