Why do unbelievers ignore and even deny this fact?

Why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined? ... But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth?… Why then is not every geological formation and every stratum full of such intermediate links? Geology uredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against my theory.(Charles Darwin, The Origin of Species, Oxford University Press, New York, 1998, pp. 140, 141, 227)