Well if you are Charles Leclerc, it's because you were born there and that is where your family live, but for others, there are a number great of reasons.
Monaco is a principality and if you live there for more than 6 months of the year you are not subjected to Capital or Wealth taxes. Paparazzi are not welcome in Monaco so it is easier for the rich and famous to move around without being bothered. And, of course, it is a "Billionaires playground" on the French Riviera.