Because they're awesome. And the more awesome something is, the more people want it. And the more time that passes after their release, the harder it is to find anywhere else other than from someone who already has it. It's basically like this:
The rarity of finding one after it's "sold out" + the coolness + peoples' increased desperation and determination = a willingness to pay more money on the only other place it can be bought (the secondary market)
And the more people are willing to pay, the higher the price that sellers ask. A good example would be this year's SDCC Guardian Predator. He started out at $350 on eBay. Now he's at $700-$800.
The rarity of finding one after it's "sold out" + the coolness + peoples' increased desperation and determination = a willingness to pay more money on the only other place it can be bought (the secondary market)
And the more people are willing to pay, the higher the price that sellers ask. A good example would be this year's SDCC Guardian Predator. He started out at $350 on eBay. Now he's at $700-$800.