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.
One of the factors that makes the level different between Paris Saint Germain and other teams in Ligue 1 is the financial condition. Paris Saint Germain can be very free to get the players they want, even though they have to get them at a very fantastic price, while other teams cannot have it. Even other teams have to sell players who play well to get a profit, that is a very striking difference. It is undeniable that money has an influence on a team, therefore football and business cannot be separated and must go hand in hand. I think this will continue to happen when there is no change experienced by other teams, such as another team being acquired by a prince.