Because if part of their nature is to crave chaos, it also seems to be part of it to keep their word. For better or worse, everyone who made a bargain with the orb got what they asked for.

[And unlike some stories Sabriel heard as a child, the orbs have so far adhered to the spirit of the wishes, and not just the letter.]

And they have not been... creative in how they interpret wishes, although some who struck bargains with it might not have understood what they were giving up in exchange.