Considering that most Americans adopted an isolationist sentiment following WWI, how was Congress able to justify passage of this act? I remember reading somewhere that in 1933, 39% of US college males said they'd refuse to fight if drafted, so I'd imagine that their attitudes didn't change in the years that followed.
The principal argument -- and the same one used for the Two Ocean Navy Act of the same year -- was that the United States needed to have a strong military in case it was attacked. At the time, the United States military was extremely small in numbers when compared with the militaries of the warring powers.
Reconsider your assumption.
The end of WW1 was 22 years earlier, 1933 7 years earlier, before the rise of the Nazis, the latest Sino-Japanese War, the start of a new war in Europe, Italian aggression, the Spanish Civil War, &c.
I can see a lot of reasons for their attitudes to change.