Why are fathers missing in Mark 10:30?

Mark 10:29 mentions father, but Mark 10:30 doesn't.

   Jesus said, “Truly, I say to you, there is no one who has left house or brothers or sisters or mother or father or children or lands, for my sake and for the gospel, who will not receive a hundredfold now in this time, houses and brothers and sisters and mothers and children and lands, with persecutions, and in the age to come eternal life.—Mark 10:29-30 (ESV)

A parallel in Matthew 19:29 (ESV):

   And everyone who has left houses or brothers or sisters or father or mother or children or lands, for my name's sake, will receive a hundredfold and will inherit eternal life.

And a possible clue in Matthew 23:9 (ESV):

   And call no man your father on earth, for you have one Father, who is in heaven.

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Mat 23:9 answers your question.

When we leave behind unbelieving family and inheritance to follow Christ, we gain a hundredfold from fellowship in the church, where there are no fathers. "in this time" makes it clear that the reward is in this life, accompanied by persecutions.

In the world to come we gain eternal life.