Because there’s grass and grass. 4G pitches mimic a grass football pitch, which were seeded with a large proportion of hard-wearing but coarse ryegrass, and mown to 50 to 75 mm. That worked fine with a foot diameter football. Hockey on grass used to be played on the outfield of a cricket pitch, with a much greater proportion of fine grasses that could be mown much closer, to 25 to 40 mm. That was perfect for a hockey ball, which was basically a cricket ball painted white. The best grass pitches, like Spa Ground in Cheltenham, or Imperial Grounds in Bristol, were fantastic to play on.
We could no more play hockey on a football pitch in 1973 than we can on its plastic equivalent in 2023.