Do you have any proof for this at all? The most anticipated boxing matches usually get over a million pay-per-view buys, not including the tickets sold for the actual arenas.
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Yes, there are literally more than 80,250 (MetLife stadium's capacity) fans of boxing in the US. However e.g. the Kingdom Arena (location of Fury v. Ngannaou) seats about 30,000. If there were a 17 week boxing match season in the US, it clearly would be a much smaller draw than gridiron football, and that doesn't even include the college game, which has 8 stadiums that seat over 100k (and 6 more that are larger than MetLife).