Some of that has to do with saturation. With 92 teams above the “professional” line (and at least that many below) in English football and most small (by American standards) cities having 2 teams, stadiums aren’t that big. But that there’s still tons of people at the games on Saturday. Birmingham is the 2nd largest city in England, and has about the population of Tucson, and has 2 teams with 30,000 seat stadiums. Comparing stadium sizes internationally is very apples and oranges.
London alone has about 12 professional soccer teams. I’ve been to one stadium in London, Charlton Athletic, which just made it back to the Championship, and that’s about 22,000.
So let's look at population size. Birmingham, England has a population of 2.7 million people and fills, as you say, 2 stadiums of 30K seats. You'd think it'd be tough for much smaller 116K population size Tuscaloosa, AL to fill their 100K seat stadium (Bryant Denny Stadium for Alabama football). Obviously, most of the Bama fans going to the game come from outside of Tuscaloosa.
And it's not like the U.S. doesn't have plenty of saturation of American football with the NFL (32 teams) and other professional leagues like the UFL (merged with XFL and has 8 teams), GDFL (29 teams), and RPFL (12 teams). Combine that with 136 FBS college teams. To me that seems like tons of saturation of both professional and high college level teams (209 teams). Not to mention saturation from other sports like basketball, baseball, and hockey to spread out the fandom. As well as saturation of college fandom for lower level colleges (I attend my much smaller alma matter's games more often than I attend Bama's games.)
Yet even with all of that competition against big college football games, our one country can fill eight 100K seat stadiums on college football Saturdays.