It's risk-aversion. Think of it this way: if drive-models are assigned to Macs in a round-robin fashion, then if there are three drive manufacturers and one has shipped a buggy drive, at most 1/3 of customers would be sending their laptops back for repairs. On the other hand, if their only supplier gave them buggy drives, then everyone would be sending their laptops back.

It also lets them have a stronger negotiating position to drop a crap supplier. They can only maintain that negotiating position, though, if they buy drives from all of them; if they only used drives from whoever was the at-the-time "best" supplier, then if that supplier started to suck, all the others would demand a king's ransom for the "privilege" of switching to them.