The World Cup is different. In previous World Cups, Brazil and Italy both successfully defended their titles, although that was in the distant 1930s and 1960s. In order to defend the title, we must first maintain our own strength, even better than the last competition.
However, the World Cup is held every four years. If you want to defend the title, the main players who won the championship last time are four years older, and their physical strength and state are hard to compare with last time. At this time, fresh blood must be added. However, if the younger players can't keep up, their strength will decline, and defending their titles will cast a shadow.
Another reason is that since you won the championship last time, your main competitors will conduct in-depth research on your tactics and staffing. There is no secret, you can only compete by strength and luck. If luck is not on your side, or your strength is not as good as before, it is impossible to defend your title.