I wish I could put my finger on why sports in the US don't do as many chants. You see it in some fashion: Duke basketball fans at home have their own thing, the Vikings have their Viking clap, J-E-T-S, Fly Eagles Fly, college football student sections, etc. but outside of Cameron none of it is almost all game long. Euro/SouthAmr soccer simply destroys the US in terms of fan participation. Yeah they get to bring in flares and banners which adds to the hype, but still ... you just can't compare.
So ... why is it? Are they drunker over there? Less self-concious? I could never see an NBA crowd singing all game long, they are way too into "being seen" in those first few rows. Are US fans "embarrassed" to sing along? I think it may be that. Then again, MLS crowds chant.