if moshe led us out of egypt, into israel, and is a great man why isn't he considered a forefather.
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The forefathers are the great grandparents of every single Jew, however Moshe is not the great grandfather of every single Jew.
See Ohr Hatorah by the Tzemach Tzedek
There are only three fathers because they embodied three characteristics that all Jews have (as an inheritance from them, because they are the source of all Jewish souls). But we do not have the characteristics other Tzaddikim (even like Reuvain, Shimon, etc. from whom we descend)
As Gershon stated, the forefathers are our physical ancestors as well as our spiritual founders. We could just say we have one forefather, Ya'akov, or we could say we have 12, his sons. Or we could trace back to his father, Yitzhak, who taught him his tradition. Or we could trace back to his father, Avraham, who established the tradition. It's better to go to the source.
In addition, sometimes "Avoth" is used in reference to everyone in the wilderness after the Exodus.
But I assume the question is why "Avoth" generally is referring to Avraham, Yitzhak and Ya'akov.