Most people don't choose reincarnation. Reincarnation finds their way to them. Perhaps they did something great in their life. Or maybe truck-sama wasn't looking where it was going. But without fail, those who reincarnate are destined for something great. A new life where everything changes for the better. But... Reincarnation isn't always the greatest thing. Sometimes you just draw the short end of the stick. Di Tou just happens to have drawn the shortest stick. Pulled out of his life randomly. Taken by a crazy old eccentric man not as a disciple, but as a body double?! And a janitor body double no less?! Grasping a system to find... It's a JANITOR SYSTEM!? Truly, how unlucky can one get?
Nowadays, the difficulty or 'gate' between stages came more in one's comprehension ability.
For instance, the ability to find the first strand of Qi. Of course, this gate alone would stop thousands of cultivators from being able to advance into the next stage, but it was already significantly easier than the past, in which advancing a stage as early as Golden Core would cause a sudden death… Or advancing into Qi Condensing might cripple one's meridians due to a minor mistake.
This is all to say that the spike in power between stages of cultivation wasn't very immediate, rather it was the spike between the peaks of two cultivation levels that was much more plain to see, as well as the unique abilities that certain stages would offer.