Why is the NFL called football?

Because it's played on your feet. When general football was coming out of the British universities in the mid 19th century it was called football because the games were played on their feet, as opposed to the games of nobility which were played on horseback. Many types of "football" came out of this period three of which are still played today. Rugby Football, sometimes called Rugger, association football shortened to soccer, and gridiron football which would become popular in North America as Canadian and American Football.