They may be closer, but the football field is still significantly further away. Soccer fields are over 60 feet wider than American football fields. Add to that fact that you have to account for sideline accommodations like benches and so forth, and there’s no way that you can get the seats closer for American football. I’m not sure if I’m making sense here but that’s how I visualize it.
Let me give you some numerical context.
Per the rules of soccer, field-size can vary (kinda like baseball). The minimum size of a soccer pitch is 50 yards wide by 100 yards long. The maximum size of a soccer pitch is 100 yards wide by 130 yards long. MLS requires at least 70 yards wide by 110 yards long.
An American football playing surface is 53.33 yards wide by 120 yards long (including end zones).
So when Joe Robbie built the stadium to host both football and World Cup, he went for the maximum size. The length of the field was fine (didn't have to add much to the end zones to go from 120 yards to 130 yards), but he made it MUCH wider to accommodate the 100 yard width favored in international play. Even when you account for the non-playing parts of the sidelines, which are bigger for football than soccer, it was still a wider field than he would have built if he was only building for soccer.
Here are the most recent purpose-built soccer pitches in the US:
Nashville (2022) - 75 yards wide by 120 yards long
Columbus (2021) - 75 yards wide by 120 yards long
Austin (2021) - 75 yards wide by 115 yards long
Cincinnati (2021) - 75 yards wide by 110 yards long
Minnesota (2019) - 75 yards wide by 115 yards long
DC (2018) - 75 yards wide by 115 yards long
LAFC (2018) - 75 yards wide by 115 yards long
Orlando (2017) - 75 yards wide by 120 yards long
And here are MLS teams that play in football stadiums in the US:
Charlotte (2022) - unknown pitch size (football stadium opened in 1996)
Atlanta (2017) - 75 yards wide by 115 yards long (football stadium opened in 2017)
Seattle (2009) - 74 yards wide by 114 yards long (football stadium opened in 2002)
New England (2002) - 75 yards wide by 115 yards long (football stadium opened in 2002)
Chicago (1998) - 74 yards wide by 115 yards long (football stadium opened in 1924)
Ultimately, while New England and Atlanta are the only 2 football stadiums that contemplated football/soccer when built, nearly every field is the same width (except for evil Yankee Stadium, which actually falls below the MLS requirements with a 68 yard wide pitch, regardless of the lies they tell about it being 70 yards wide).
So the soccer playing surfaces are 22 yards wider. (75 yards vs. 53 yards, rounded) Just under 11 yards on each side. When you factor in the wider NFL/college football sidelines, it's very comparable.
The original problem was that Joe Robbie built a stadium to accomodate a 100 yard wide pitch, which has been partially addressed by filling in the sideline seats in the lower bowl (building more seating closer to the field).