If you consider the true element or immortal ascension cultivators, even well-off sects don't have that many of them, despite be severe power scale inflation in this series. You need considerable talent, luck, pills/other resources, and courage to find opportunities and face challenges to get there. So, no sect would automatically have cultivators for every nine stages of those ranks. It would be totally random what they would have. Thus there are bound to be gaps as well, since the total number isn't so high that every stage would be populated simply through brute force of statistics. On the contrary, it's like saying that if you roll a dice (d6) six times, you'd easily get every number from 1-6. That's actually quite unlikely.
To put it the other way, if a sect takes 9 random apprentices, gives them all equal resources and chances to find opportunities, will every one of them be at true element stage 5 exactly 20 years later? Or however much time you'd approximately need to give them in this series. I think not! There are no secure ways to make sure a cultivator will grow steadily and thus it's not so probably all the stages in the upper ranks would be populated. Especially since there's a bottleneck between those ranks! So, folks would slightly accumulate toward the later stages of each rank, as not all of them smoothly make it to the next rank.
The author thought he wrote this smartly, but he didn't. Also, if someone reads this whole comment, you wasted your time! The time would have been better used cultivating.