Thats just how 3rd party/generic lenses make money. Many are very favorably comparable in quality to name brand lenses, with enough research you can find reviews that back up those claims, and there are a few 3rd party fanboys that lurk forums that will tout their superiority till the end of time. They are also priced to compete with name brand used and refurbished lenses, and always cheaper than the new name brand lenses. When a potential buyer like you is looking for a specific lens, they are always tempted with the dilemma of "hey I can get this off brand lens for just a little more than a used Canon version, and some websites rave over how good this lens is in comparison with the Canon too, so it makes sense to buy the new off brand one right?" The problem comes with holding their value. A decent Canon or Nikon lens always holds its value very well. 3rd party lenses never do, there is no demand for them compared with name brand. None of these lenses are inexpensive, it comes down to wise investment of your money. Buy 3rd party lenses used never new.
Do some simple math, man. Get out of here with your talk about lenses holding their value. I can either buy this lens for $1,000 or buy the Canon version for $2,500. Both will lose about $400 in 5-6 years' time, but the difference is that with the Sigma, I still have $1,500 in my pocket to play spend however I wish, whether that means that I can upgrade bodies that much earlier, or whether I can buy something not photography related. I'll tell you what: you buy the Canon version, I'll buy the Sigma version and put the rest of the money in a CD. Let's see who comes out ahead.
Also, again, I think that you're highly overestimating the depreciation. Take a look at ebay, for example, and look at used lenses with completed listings. The used prices are a little bit more optimistic than you make them out to be: