As to the immediate, practical question, I think some of the contributors have made efforts here to prove options with the specs you desire are in fact available. There remains the caveat reliable, but that's difficult to judge, especially with newer products. If you go by survey results, Apple and Lenovo ThinkPads win by a mile, but they charge a hefty premium. (And both brands 'build quality may not be what it was even a few years ago.)
I do think, with notebooks, you generally get what you pay for. I work in university IT and always share that maxim with prospective buyers. Computer components may be commodities, but their integration in a notebook, and the general fit and finish of the whole, is more critical with a product that's going to be exposed to all the outside world's bumps and buffeting. So if we know that the minimum cost for the components you desire is X, and the minimum assembly, labour, etc costs are Y, then the premium Z for reliablity and build quality will, I suspect, always remain significant for notebooks.
I suppose your question could be rephrased as
1.) Why hasn't X fallen enough so that even with a sufficient quality premium Z, X+Y+Z+profit will be < $500.
or
2.) Why hasn't technology or manufacturing techniques dropped Z more significantly?
Aside from 2.5-inch hard drives [edited: and memory, duh], notebook components aren't really sold at the retail level in anything like the volumes of desktop components (for obvious reasons). It's harder for geeks to engage in our hobby of tallying a list of retail components and comparing them to an OEM's finished product price. Sites like iSuppli do teardowns of high-profile products (eg Apple) and estimate X for us, which enables investors to obtain the figure they care about (profit).
If you feel companies aren't entitled to large profit margins, than that figure may fill you with rage or contempt. But in many markets, it's difficult to obtain a product that has decent Z (and thus one hopes high quality and reliability) without being accompanied by a hefty profit margin.
Many people compare Apple computers to luxury motor cars like BMW, and if you permit me to apply that analogy, I suppose you're wondering why there's no equivalent to the cheap but dependable Japanese car? Someone more familiar with engineering and manfuacturing could perhaps supply an answer.