This is actually a common problem with all rare colors, especially reds. The thing is, though, most people don't consider BLONDES to be a rare color type . . . but they are.
Most blonde adults are not, shall we say . . . natural blondes. Or at least, not blondes that are THAT light. Blonde hair tends to darken naturally with age. The colors that people are trying to achieve via wigs and extensions are extremely rare in nature. They are difficult to duplicate convincingly, because they only tend to appear in real life on people with the aid of significant chemical intervention from a gifted stylist. (Not to say some folks don't have naturally light blonde hair as adults; it's just very rare, just like natural red hair. Most adult "blondes" get some color assistance somewhere.)
The colors you'll see in most wigs, therefore, mimic these colorists' creations and NOT the patterns of natural blondes. Why? Because that is (theoretically) what most women want, because it looks expensive, and is the fashion of how women tend to wear blonde hair: slightly darker at the nape, highlighted on the top and near the front at the face, and usually with a slightly darker root (to mimic the look of getting your hair dyed). ;)