Presumably because he seemed that much more like a god than like a man.
Our present understanding of heredity was not yet available, and absent that, the proportion between the genetic inheritances from father and mother was pretty much anyone’s guess. In the Eumenides of Aeschylus, the god Apollo claims the father’s share is 100%! (The chorus of Furies does not buy it.) Sterne in Tristram Shandy (Vol. 1 [1759] Chap. 2) mocks the somewhat similar theory of the spermatozoon as containing a miniature but complete version of the person to be begotten, the homunculus.
Even with the benefit of modern understanding of chromosomes, meiosis, and fertilization, the notion persists that one can be one-quarter Czech (or whatever), because exactly one grandparent was supposedly pure Czech, persists despite the obvious arithmetical fact that the number of chromosomes in our species’ somatic cell nuclei, forty-six, is not an integral multiple of four. This obsolete notion is even written into current law, with current real consequences, as applied to membership in recognized Indian tribes within the United States.