Within a foursome, it's possible that all 4 golfers will come from different households...this might be their reasoning. Once you multiply that by the number of groups that can tee off in a single day over hundreds of courses the number of possible "exposure events" can be high, then add in a few drinks, no mask wearing etc...
Keep in mind, in the winter you could not go downhill skiing for the longest time even though that is a solo activity, with mandatory social distancing while wearing a face covering.
Why should golfers golf when skiers could not go skiing. The risk from virus transmission while skiing is way less than golfing.