Why do the five consciousnesses transform only when approaching Buddhahood?

"The sixth and seventh consciousnesses are transformed on the causal ground, while the fifth and eighth consciousnesses are perfected on the resultant ground" means that the sixth and seventh consciousnesses must undergo continuous transformation throughout the process of cultivation, whereas the fifth and eighth consciousnesses are only transformed and perfected at the very end when Buddhahood is attained. The process is the causal ground, and attaining Buddhahood is the resultant ground. In this process from cause to result, the fundamental transformations of the sixth and seventh consciousnesses must be experienced three times, while their gradual transformations must be experienced countless times. Why must the sixth and seventh consciousnesses undergo countless gradual transformations before wisdom can be perfected and Buddhahood attained at the end? Because the sixth and seventh consciousnesses are endowed with all ignorance and afflictions, they need to continuously remove ignorance and sever afflictions through cultivation, constantly purify the defiled nature of the mind, and at the same time continually increase merit and pure wisdom.

From this it can be seen that although the sixth and seventh consciousnesses are endowed with the nature of afflictions, they will ultimately also be endowed with the nature of purity, the nature of bodhi, and the nature of nirvāṇa. This shows that the sixth and seventh consciousnesses are endowed with both wholesome and unwholesome mental factors (caittas); neither of them possesses only a neutral/indeterminate (avyākṛta) nature. Otherwise, cultivation over such long eons would be impossible, and there might not even be any need to cultivate.

As for why the five sense consciousnesses do not undergo continuous transformation during the process of cultivation: it is because the five sense consciousnesses are completely subordinate to the manas; they are the tools used by the manas to discern and process sense objects, entirely serving and obeying the manas, without any independence of their own. Therefore, the five sense consciousnesses have no wholesome or unwholesome nature, do not need to cultivate goodness or sever evil, and thus their mental nature undergoes no transformation, nor does it need to. However, because the merit and wisdom of the sixth and seventh consciousnesses are not yet complete and their stage is low, the functions of the accompanying five-consciousness apparatus that He possesses are also low, and the undertakings He carries out are therefore not perfect and complete. When one is on the verge of attaining Buddhahood, the merit and wisdom of the sixth and seventh consciousnesses become perfect, their virtues complete, and their mental nature perfectly luminous; the accompanying apparatus He uses must naturally be upgraded to the highest grade, capable of assisting the sixth and seventh consciousnesses in perfectly accomplishing all undertakings without obstruction. In this way, the five sense consciousnesses must be matched and consistent with the virtues of the manas, transforming from the nature of sense consciousness into the nature of the wisdom of perfect accomplishment (krtyānuṣṭhāna-jñāna), with extremely sublime virtues.