The same also seems true in football/soccer for 
    reasons that I don't quite understand. It wasn't 
    ever really my sport, and I don't get why those guys 
    are so shredded everywhere.

They're not just running in straight lines. They're making all kinds of lateral cuts and contorting their body in various ways as they jockey for position with other players for contested balls, on set pieces, etc. Those things rely heavily on core abdominal strength. They also use their arms quite a bit, which is why they are not just withered vestigal appendages.