Yes, I believe that PC home brewing and white-boxing still exist, but on a much smaller scale than they used to. I think there are still hard core aficionados out there, most of them gamers, who will always prefer to build a monster gaming machine for a fraction of what it would cost them off the shelf at a store. There will always be people who prefer to build their own PC's.
After research it appears that pc home brewing and white boxing does still exist these days. There is evidence to support it exists and is still used widely. Various articles support this theory. It is not as common as it once was but it is still something that is done.
While there are many more great quality computers on the market today than ever before; There is not always the perfect machine for every ones uses. For instance many currently marketed gaming computers lack what the hardcore gamer really wants. In these cases it can be a much better option to choose the exact components yourself that you want to make the perfect computer for your specific gaming needs.
PC homebrewing and white-boxing has become less prevalent over the years, but that does not mean that has gone away. While the average consumer would much prefer to buy a pre-made OEM PC in order to save time and stress, there still exists a fairly large group of people who would much rather prefer to build or simply have total control over their computers. A common group in which this still exists can be seen in what is considered the PC Gaming market as those who are referred to as "PC Gamers", those who play games on a PC. This group commonly builds PCs from scratch rather then purchasing and OEM Machine in order to save costs and have control on repairs and upgrading in the future.