This could be seen from the point of view of divinity cohesion. After all, the environment of the world of Gods was not so good that one could find high-purity Power of Laws everywhere. One could only rely on oneself if one could not obtain it from the outside world.
Thinking about it this way, after advancing to the ninth rank, if one wanted to advance further, one would need to cohere a pure, single law and sublimate it to a level that was sufficient to cohere divinity. Only at this level could one cohere a divinity and advance to the Demigod level.
One could imagine how difficult it would be. After all, cohering a law to that degree was a significant test of one's aptitude and talent. Apart from the high-purity Power of Laws, one also required sufficient world's power, not to mention cohering to a divinity. The difficulty was imaginable.