Twice less Gold medals you mean, because the total medals are pretty much the same in all their last Olympics.
This happens, when you don't have superstars like Phelps and Lochte to anchor the team. In Tokyo, Dressel was this superstar, who held them, but this time we saw what happened when he wasn't on form. Their women were the stronger side, but they are also losing positions in some events.
I think this is maybe the first time in history, where the majority of the current swimming superstars are not American - Marchand, Pan, Popovici, Milak, Mcintosh, Titmus, McKeown, with the only exception being Ledecky of course. Unless Dressel somehow finds his mojo again.