Volundr: there's a fair amount of downtime in football, yes, but during that downtime a lot of strategic adjustments are being made, so for a football buff it's not tedium but a crucial component of the game. The action that does occur is always meaningful - the majority of offensive plays result in scoring, and the offensive switch-off between teams is much more structured, so even when there's no touchdown or field goal, the lack of points is itself significant. Compare this to soccer or hockey where both teams are likely to take a ton of shots on net and the success rate is extremely low - over the long term, having the offensive advantage translates to more goals, but often time it feels like dumb luck who wins the game when both sides had 20 amazing offensive plays and the game still ends 1-0. The only other sports I can think of where every action feels as meaningful as it does in a football game are individual sports like tennis. Basketball can match the intensity in terms of everything that happens on the field being likely to have an actual effect on the score, but basketball is so fast-paced and high scoring that I find it somewhat hard to care until the last couple minutes. Nothing in all of spectator sports matches the excitement level of seeing a really, really good offensive play in football. Football strikes a zen-like balance between high scoring games and keeping each play meaningful, and between exciting on-field action and structure/timing.
Plus, every time Dallas loses I get to mock all my friends who are Cowboys fans. I'm a Cowboys anti-fan more so than a fan of any particular team. Since leaving upstate NY I've stopped really following the Bills, so I guess my team is whoever's playing the Cowboys next
And yeah, Americans tend to be more interested in rivalries between American teams than between the US and other countries - we're too invested as a nation in delusions of grandeur, we can't put them to the test by competing against, and potentially losing to, other countries, so we pick a sport that nobody else really plays (or at least, nobody else plays well enough to compete with our teams). Football (American Football, that is) also has the 'armchair quarterback' appeal - I think it's more appealing than most other sports to people who have never played it. Plus, it makes for great highlight reels
