Eldar are your classic, pre-Tolkein Elves - completely self-serving bellends, with absolutely no regard for your well being. Their boon comes at a price, and they never end up short changed.
For reference, watch the really rather wonderful 'Jonathan Strange & Mr Norrell'...see what happens when you start to bargain with the Fey.
Or you can read it, if that's your thing!
Remember folks, Ghaz is Eldrad's baby. He did all that to preserve a single Craftworld, on a hunch it might run into danger. The dickhead.