The same course, different days
Because a full field cannot play at once, tournaments split players across morning and afternoon tee times, and often across two starting holes as well. If weather shifts during the day, players are not competing on the same course at all in any meaningful sense, even though the scorecard treats their rounds as directly comparable. A calm morning followed by a windy afternoon can separate two equally skilled players by several shots before either has hit a poor one.
How much conditions can move
Wind speed and direction, rainfall, and green firmness can all change meaningfully within a matter of hours, particularly in coastal or changeable climates. A soft, receptive course in the morning can firm up considerably by the afternoon as it dries, altering which shots are attackable and which pins are fair, so scoring conditions are rarely as stable across a single day as a single leaderboard implies.
Why the format persists anyway
Organisers accept this variance because the realistic alternatives are worse: playing every round in strict sequence would make tournaments impossibly long, and constantly adjusted scoring to compensate for conditions would introduce its own arguments and inconsistencies. The unfairness is tolerated as a known, bounded cost of a workable schedule, and most players and observers treat it as an accepted feature of stroke-play golf rather than a flaw worth redesigning.