If Matthew 23:9 says to call no man 'father' on earth, why do we call priests 'father'?

You have to read it in context, because it is a figure of speech. The whole passage is about the Pharisees, who are hypocrites: they say one thing and do another, yet they demand respectful titles from the people. 'Father,' Abba in Hebrew, is one of the most respectful ways to address someone in authority; Jewish children call their father Abba, and it was used in the Old Testament. The Pharisees sat in Moses' seat and amassed all authority to themselves, mistreating the people while still demanding to be honored as fathers. So Jesus says do not give them that honor; obey what they teach from the law, but do not imitate them, because they are hypocrites, and because your real spiritual Father is God in heaven, not these men who exploit the natural relationship of father and son to exalt themselves. When we call a priest 'father,' or a bishop 'your Excellency,' that is a recognition of the hierarchy God established and of the authority they have over their flock; there are proper terms of respect for those in higher office. The line Jesus draws is with hypocrites: for a man like Father James Martin, I would not use the title, because he does not deserve it, and that is exactly where Jesus drew the line too.