For me, competitiveness is largely about how directly players can influence and outwit each other. In tennis, psychological pressure is just one element alongside direct tactical interaction. In a parallel sport like golf, that interaction is much more limited, which makes it feel less competitive to me. I can see the counterargument, though: because golf has fewer direct competitive elements, the psychological battle may become more significant in its own right, but I personally don't value that so highly; its something that must automatically be done anyways in sports where you are directly influencing each other alongside all the extra baggage of strategy they bring!