Well solely because if the media are to be believed, a few clubs had a look during the summer (Arsenal among them) and weren't willing to pay his release clause. I suspect his true value is around the price Everton paid originally, or a bit over. So when you factor in us wanting rid, him not performing (all in this hypothetical, obviously), cutting a few million of the actual price he's worth when he's playing well sounds right to me. Big clubs selling players they don't want very rarely get the bulk of their money back, especially if they overpaid.
Or that could all be a load of bollocks. After all Andy Carroll has transferred for fees of £52m odd in his career.