Well, the quick answer is we do it that way because we have evidence God wants us to. Scripture consistently calls God ‘Father’. When Christ prayed before witnesses He called God ‘Father’.
Slightly more thoughtful answer: the image of our Mother in Heaven evokes different images than that of our Father in Heaven. We get a different impression of what God must be like. It looks like God thinks the masculine image is better for us to uses, which implies it’s more accurate.
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