In part we can look to the prevalence of collegiate sports programs across the nation as an alternate form.

New York City for instance at one time all had Columbia, Fordham, NYU, St. John's, and just up the river, West Point, fielding teams with Yale, Princeton, and Rutgers just outside the City, attracting young men to play, and everyone else to watch.

To get a sense of just how many schools still today are playing NCAA Football this map shows their location. And there are a lot across all 3 divisions

So we can say that college sports drew much of the early interest, development, and focus of competitive sports, that other nations saw in club sports. Gridiron football was born and nurtured on the campuses of the Ivy League after all.

Though there are a plethora of more independent professional, and semi professional teams across the North East and Mid West. Many of who either couldnt sustain themselves, closed during the Depression, or were left out in the various mergers and consolidations that ensured the early NFL could make a profit.

Its wiki but take a look at the list of defunct NFL teams and their geographic concentration: https://en.wikipedia.org/wiki/List_of_defunct_National_Football_League_franchises