This question is actually not worded correctly--it should state "why is the possibility of a human female suffering from SEVERE hemophilia rare?" Women who are carriers frequently do themselves have mild or moderate hemophilia, but only rarely have severe hemophilia. This is because they have two X chromosomes, one from each parent and only one of which can be functional in a cell. Normally this X chromosome inactivation is random, with about half of the X chromosomes from the male and half from the female being inactivated--this leaves some capacity to produce functional clotting factor, but not normal amounts. In very rare cases a woman who has one gene for hemophilia can have severe hemophilia through a process called non-random X chromosome inactivation or Lyon's Hypothesis. In this case, all of the "good" X chromosomes are inactivated leaving only the X chromosome with the hemophilia gene, preventing the female from producing any functional factor.