That's what makes this league not so interesting because PSG can claim the title much earlier before the season ends so the league competition is not so competitive. It is difficult for any team to break PSG's dominance and that is not at all the competition that football lovers expect. To be a competitor for PSG in the fight for the title, it takes a balanced player strength and that is not possessed by Marseille or Monaco so it is difficult for both of them to compete with PSG.
If you look at the Marseille team when facing their opponents they are able to win the match with a fairly large score but when they want to compete for the title with PSG they actually lose their chances, both Marseille and Monaco are both seven points behind PSG so it is difficult to catch up on the points.
Because PSG's level is much higher than all other teams, and they can have a hard time playing against each other and they lose points, and PSG at this time does not feel such a big competition. Ligue 1 could be more competitive if other teams could find the same funding as PSG, but those who have big money are more interested in investing it in the EPL teams, so the competition there is much higher, and Ligue 1 is a championship of one leader, as it happens in weaker leagues.