That's somewhat surprising to me. There's so many MMOs out there that have failed for one reason or another that you'd think someone would be trying to salvage their sunk costs by selling the engine.
Despite the competitiveness of the MMO market, there are still a lot of companies trying to break in, so you'd think that there'd be a decent sized market for a product which would shave years off of the development time.
I wonder if part of the problem comes from most MMO companies clinging to life for as long as they can, and then not having the money to rework their technology into something suitable for a third party. Certainly, there have been quality(from a technical standpoint, anyways) games that got released, got a few rounds of post launch patching, and then died from lack of users. I'd bet that with a bit of polish, that sort of proven technology would be a moneymaker.