The reason is simple.
First, every few years, Immortal Sects regularly select qualified disciples within their jurisdiction.
If there were any qualified candidates, they would have already been recruited, not overlooked in the wild.
Those who get overlooked are mostly those with insufficient talents or temperaments; no one wants them, and they have no choice but to become independent cultivators.
Such people are naturally disdained by the Immortal Sects.
Second, independent cultivators are a mixed bunch, and many among them are demonic cultivators with dubious character.
Recruiting such people would only tarnish the sect's morals and reputation.
Third, outsiders are not trustworthy. Who knows if there are spies planted by other sects among them?
If a spy were recruited, whether he steals the sect's techniques and magical treasures, communicates with outsiders, or turns traitor in times of crisis...
For any sect, the harm could be unimaginable.
