On the malicious side, the reincarnation of the polluted land needs a living person as a sacrifice. Who wants to be the victim? Moreover, the strength of filth after metempsychosis is determined by the strength of sacrifice (mainly reflected in bloodline ability). If a movie level wants to display its 890% strength, it must find someone who is not weak. No one is willing to make such a sacrifice voluntarily. If forced, it will be remembered for thousands of years.
On the restrictive side, the strength mentioned above is one thing. In addition, this technology must be controlled by performers. If the performer is disloyal or prepared in advance (for example, orochimaru put a spell on the first and second generations in the early stage and forced them to give orders), this is asking for trouble. In addition, metempsychosis technology also has an upper limit. For example, the man who endured the knife with seven people and was reincarnated as a blaster will blow up all the detonators on the knife when he is surrounded. Teammates told him that even the metempsychosis body could not bear it (perhaps the power of sacrifice was too weak).
As for why Spot knew about the reincarnation of dirty soil, the third generation was too young when the second generation died, and the fourth generation led troops to fight when Spot died (referring to World War I without Qiao Kun). During this period, a large number of Bai Jue who were good at collecting information appeared. He probably wants to know how many hairs three generations have, and he is really happy. Anyway, he has enough time.
Why didn't Ying Si kill orochimaru? At that time, orochimaru was already white, and his control over chakra had reached its peak. Orochimaru still gave them the initiative (if you add magic, it is estimated that there are four puppets). The second generation tried to kill orochimaru, but was suppressed by orochimaru. He was able to get rid of orochimaru's first brother. He felt no need to return to Kan Kan and told his story. The third brother sees sex and has no motivation.
All people who can become Huo Ying are open-minded. They were summoned from death, and 80% people thought that "after helping these young people deal with the mess, they would go back happily". It is much better to fulfill their wishes and ascend to heaven than to live without ghosts.