I think (with zero evidence) that the market is partners of gamers. So you'd always wanted "mini X" and your wife/husband/mindslave decides to surprise you for your birthday or whatever. As a one off purchase as a gift (by someone who isn't doing the collecting directly and maybe factors in more realistic costs) whilst expensive it's less of an issue. Think of it more like the purchase of an ornament or piece of jewellery type thing.
I think those of us doing day to day collecting know that eventually things do re-appear, so it's only once in a while where I'll go over the odds. My Eldar Phantom Titan springs to mind, that was the second one I think I've seen on ebay ever that was certainly original and in the UK. Some extra Beasts of Nurgle, nah I'll wait until some eventually turn up at more like £20-30, not paying ~£100.