Luck or karma is always been the hidden factor of geniuses. It exists in a corporeal yet illusory way. Good work brings good karma and evil gets retribution, people believe it. But it has a very low effect of cultivators, yet nonetheless, it has. Cultivators at higher realms can alter fate and change destiny, yet there is a limit to it. In the end, fate will change itself and the cultivator will found that they were attached to higher karma or a big event that can destroy them. Fate is also like an invisible currency.
Without enough luck, you might miss a godly weapon, even if it's lying beside you. So, Fate, Destiny, Karma, Luck, all these things are still the most alluring things to cultivators. Alas, No one has ever said that they became master of destiny. (Don't mind this paragraph, its kinda crap)