The oft repeated reason for lack of Americans was a very low prize fund, and high travel expensive.
Maybe someone can resurrect the travel costs, my recollection is that if an American traveled and then won first prize, you could make more than travel expenses.
I think even Arnie was just playing for the honor and glory in his first in 1960.
Obviously prize monies increased greatly through the years.
Though WWII precluded play for many years, Nelson played but twice, a 5th in 1937 and amazingly a T32 in 1955.
Hogan played once and won in 1953. Almost certainly paid an entry fee and he won £500. He won 8 other majors from 1946 to 1953, posted other major top 10s in the 1960s. Played in The Open only once.
Snead played 3 times T11 in 1937, won in 1946, and T6 in 1962. He won on the US tour as late as 1965. Snead was a guy that followed the money.
Nicklaus won but £ 5250 in 1970. Even then it was mostly for glory and honor, but some might argue it was for increased endorsements, etc. by 1970.
In 1970, Masters winner was about $25k, US Open about $30k, Open was £5250, US PGA about $40k.