The Majors are the most important of the tennis tournaments in the world - but by far not remotely regarded as the only tennis tournaments of importance. Fact is the players themselves regard the End of Year Championship as the most important one to win. The latter tournament only allows for the 8 best players in the World in for instance in 2023. They compete throughout the year to ensure that they qualify for the said tournament.
As to Monfils - he plays only show tennis nowadays - and virtually never survive to play in Rund 2 of tournaments. Fact is he is invited by the organizers to play in some tournaments inclusive of the Masters.
However, in this case he was the surprise winner in Round 1 and then decide he will withdraw from the tournament. His opponent in Round 2 would have been Holger Rune - who now has a walkover to play in Round 3.