I'm just guessing, but I think it's because in the "old" days, they played these tournaments at places which were primarily country clubs. So there were no stadiums as we think of them today. Newport is a club, and Forest Hills is a club.
Although having been to Forest Hills, they do have a main stadium. It's very small compared to stadiums of today, but still they have it. It's hardcourts now but used to be clay so probably when the tournament was played on grass that they used the area of the club that has the grass courts.
Again quessing...don't know the answer because while I am older, I'm not that old.