Besides being able to buy in bulk there's also the economy of scale created by the amount of money necessary for setting up shop. Massive industry for the benefit of the common man. It's like the phone company, calls don't cost much but that's only because when you divide the setup and maintenance costs by the number of people who use phones it's not much. If you tried setting up your own phone lines across town or to Russia or wherever you would find out real quick like just how expensive it is. Guitars are the same way, they don't cost a whole lot from a factory compared to what it takes to do it yourself because their total fixed costs (like tools/equipment) are divided among a much larger production than just for yourself, so even using the same average variable costs per guitar (theirs is actually even lower from buying in bulk, as was mentioned by Lats) it's a huge drop in expenses for the same quality product.

Natural monopoly is not always a bad thing.