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Wesker speculates that the various mutations brought on by the various viruses are not solely dependent on host compatibility, but more on the mentality of the host and the presence of various chemicals in the brain. Going back to the other games where you fought the Mr. X Type Mass Produced Tyrants, you learn that they can only be reliably made if a certain hormone is present in the host during infection. This hormone is extracted from adolescent children, especially when under intense stress and pain. If Wesker's theories are true, this means that the Tyrant is literal fear incarnate.
Not just children; the file you can find states outright that the hormone only comes from adolescent males. And they preform the surgery while the child is awake.