The time between election and inauguration of a new U.S. president—set by the Constitution for Jan. 20 following an early November vote—is relatively long. In 2020 it‘s 78 days.
A change in the highest U.S. office triggers thousands more job changes throughout the executive branch. Many appointments must then be confirmed by the Senate. Most newly-elected leaders elsewhere have far fewer jobs to fill. "France has nearly three times as many federal public servants as the United States, but just about 1% or 2% of the number of presidential appointees," a law professor observes.
The French president takes office within as few as 10 days of final electoral results.
In the U.K.'s parliamentary system, where a leadership team functions while in opposition, a change of government normally happens the day after the election. Other similar structures allow for at least a few days to hand over power.