Length is the least of it
A course can add hundreds of yards and barely change the winning score, because modern players adapt their clubs and lines to length quickly. What is far harder to adapt to is rough that punishes a two-yard miss, greens so firm that a well-struck approach still runs through the back, or fairways narrowed just enough to remove the aggressive line off the tee. These are the levers that actually decide whether a week produces low scoring or a grinding survival contest, and they can be adjusted without moving a single tee marker.
Firmness over length
Firm, fast greens reward precision and punish anything struck slightly heavy or with too much spin, which is why they tend to separate fields more than soft, receptive ones that stop a shot wherever it lands. Firmness is also weather-dependent and can change overnight, which is one reason scoring can swing sharply between rounds even when nothing about the layout itself has changed. A setup team choosing firmness is really choosing how forgiving the whole week will be.
An editorial decision, not a neutral one
Because these choices are made by people rather than dictated by the land, course setup is inescapably a statement about what a tournament wants to reward: patience and course management, or aggression and reward for good execution. A major championship that grows its rough and firms its greens is choosing to punish the small miss severely, while a tournament with generous fairways is choosing to let birdies flow and let putting decide the outcome instead.