It is no small secret that Apple products are expensive because the profits from hardware sales is partially used to fund their software side (which cost money to upkeep, but often bring in little/no revenue by themselves) such as itunes, imessage, icloud, cheaper apps on the app-store (eg: iworks suite vs ms office), iOS updates etc. This lets you do funky things when you own multiple mac products, so Apple actually has a pretty strong selling point.
Standalone PC makers have little to differentiate their products from their competitors except for specs, pricing and design (and honestly, the more they try to differentiate themselves from the air's design, the more I find their designs look like crap).
Lastly, I believe another reason is because the current impression of apple products is that they are expensive, but more powerful than windows computers (ie: you are paying a premium for quality). So no manufacturer would dare to price their own computers at apple-equivalent prices because when if you present a consumer with a windows laptop and an equivalently specc'ed macbook, they will likely opt for the apple product. So the manufacturers are literally caught between a rock and a hard place. They need to cut prices, and lower margins forces them to cut corners like using hybrid drives instead of ssds or cheap plastic casings.
