Every 5v5 sport requires everybody to defend and it's just because it's the name of the game and you can't let other team have 20% more players involved into game in a 5v5 sports by having it at 5v4, but you could do it if it was 50v49 because the difference would just be that 2%. Also, the field in football is way bigger and game lasts 90mins with 3 subs, meanwhile basketball is 48mins with unlimited subs + the game isn't on whole time, so conserving the energy much as possible is the key and since the field is way bigger as well, the coverage around the field becomes way harder as well, so there goes the defensive duties or attacking duties if you're playing either end of the field(defense/attack) and if you did, there's a guy to cover you down there if you went to the attacking zone as a defender and so on.
It's ridiclous to compare basketball to football and even more pathetic to bring up the "defensive duties". Put up a field size of a boxing ring and play 5v5 football and im sure every single player in there would try to block all the possible shots, it just doesn't make sense to bring up the defensive stuff since football has so much bigger field to play on.