I think it partially has to do with the high costs of repairs and parts, and how great the car is. Even though these Boxsters have depreciated to sub $10k, any non-minor repair can easily cost a couple to a few thousand dollars. And we're talking about real repairs, not mods where every person has a different taste for. When the owner sells the car, they want to justify that cost because the car actually performs better than those that haven't had those repairs yet. But of course, the buyers don't care much and don't want to pay more for that.
In my personal case, I got my Boxster two years ago at 120k miles for less than $10k. For the past two years, I've done many repairs, including AOS, water pump, engine mount, rear track arms, CV boots, and many others. I spent quite some money (parts aren't cheap) to get the car running better. So when I need to sell the car, I would want to price it higher than those of the same mileage but haven't had the wear and tear areas addressed yet.
BTW, OP, I was gonna post in your intro thread, but I'll just say it here. I'm really jealous of you because you've owned two cars on my dream list, namely the BMW 8 series and the Miata NA.
