To sum up why the USA come into this event as huge favourites, one stat is all that is required: the 4x400m relay has been contested in 24 previous Olympic Games, and Team USA have taken the gold medal on 17 occasions. They won gold in 2000 but had it stripped from them 12 years later, and their boycott of the 1980 Olympics in Moscow most probably robbed them of another gold.
The class of 2021 look very impressive too, despite none of their individuals medalling in the individual 400m, they were the only nation who had two representatives in the final, with Michael Cherry and Michael Norman finishing fourth and fifth, respectively. The only two occasions where they failed to take a world gold in the last two decades came at London 2012 and four years ago in the 2017 World Championships. They have won both titles back since, and will be determined to hold on to their Olympic title here in Tokyo.
The potential weak spot in the team may come from the other two members they will need to pick alongside Cherry and Norman. Trevor Stewart and Vernon Norwood ran as big favourites to win the mixed 4x400m relay earlier in these Games but came third, and Randolph Ross failed to make it through the individual heats. USA have all the pedigree and experience in this event though, and are favourites at 1/10 to win gold, but do not discount the other nations.