Security updates and the ability to run the latest OS are very different things, especially in Mac-land. Unlike Windows software which generally runs on a very wide range of versions, most Mac apps assume users are only running more recent releases. For example, Firefox runs Windows versions up to 14 years ago (Windows XP, which isn't even supported by Microsoft any more) but only on Mac OS X within the last 6 years (OS X 10.6 Snow Leopard). Couple that with the fact that it is very difficult to determine when Apple drops support for a given version of OS X because they have no published schedule and don't officially announce when they drop support of a specific version. In short, the equivalent 2008 Windows laptop is still getting security updates for the OS it came with, works with nearly all major Windows software, and can be updated to the latest version of Windows without much fuss. The 2008 Macbook is running an unsupported version of OS X, can't be upgraded to the current version of OS X, and can't run quite a lot of Mac software.
Windows vs Mac resale value plays a lot into the way Windows folks use their laptops. The reason is that they generally use a given laptop until they no longer need it. My girlfriend's laptop will be 5 years old this year and was purchased for about the same amount of money it cost you to use your Macbook for 3 years. I spent another ~US$100 to upgrade to an SSD recently which will give it quite a bit more life. And it can be upgraded to Windows 10 for free if we want to at some point. Granted, it's only used for web browsing, office suite stuff, watching videos, recording and editing voice-over work etc, so it doesn't need to be the latest and greatest. So, it's really more a different use case.
Your typical pattern of buy and sell is kind of fascinating to me as a Windows user, but makes complete sense given the comparable markets and models. The wider array of Windows laptop choices in different budgets accounts for the lower resale value of higher end Windows laptops. Most users are fine spending $400 or less on a Windows laptop for the basic browsing, email, photos, videos, etc needs. High end Windows laptop users want a new machine, not a used one. So, there's not much interest in $500 used Windows laptops... they don't serve either market segment well. In Mac-land, there is no $400-500 Mac laptop option, so anyone with only that budget has to look at used models. Thus, the demand keeps the cost for a used Macbook high compared to Windows laptops.
In the end, I think we're both doing it right for our needs :)