Why a striped baby is not the result of a white and a black parent?
The reason that there are no stripes when black and white people get babies is the fact that the number of pigment cells is the same for black and white people. What is different is the amount of pigment produced, and it will be different in the kids as well (lighter than the dark parent, darker than the light parent) See references 1 and 2 for details on the melanocyte density. Skin color is a multigenic trait, so the genetic background of both parents and their effects on pigmentation come into full play here.

Besides this effect, humans do have stripes, although they are mostly not visible. They are called Blaschko's lines and look like below:



These develop from the migration pattern of the melanocytes (and their precursor cells) in the developing embryo. They can get visible in some rare genetic conditions or by diseases. See reference 3 for a more popular explanation and reference 4 for a review on this field.

References:


The Regional Anatomy of the Human Integument with Special Reference
to the Distribution of Hair Follicles, Sweat Glands and Melanocytes
General Biology of Mammalian Pigmentation
Blaschko’s Lines
Lines of Blaschko.