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.
Lens came close to winning the Ligue One in the 2022/2023 season. They were such an amazing team that season that they lost the title by just one point. They didn't have a big budget or world-class players, but they had a disciplined and desperate squad. Since then, PSG has maintained their dominance of the French League. Having more clubs that enjoy the financial backing PSG has will be fine since they will be able to buy the best players in the market. But just as Lens came close, maybe a club can still do the same but I don't think it will be this season.