Quite suprised that the US swimming team is only chosen just over a month before the Olympics. Was assuming a team would be picked at least 6 months to a year ahead of time to allow for more intense training, team building etc for relay.
They need to peak at the right time. If the trials are 6 months out they have to peak then, then come down, then peak again for the actual Olympics. By having a trial close to the Olympics it allows an intense building phase of training over winter with the trials probably just sub-peak and then finally peaking a month later.