Why do vampires always go for the neck?

The carotid artery isn't far beneath the neck, on either side, and is one of the larger arteries accessible without taking off clothing. Vampires do not move for the neck exclusively, however. In mythology, some vampires move for the femoral artery, accessible thru the inside of the thigh, or the brachial artery, on the inside of the upper arm.

In reality a vampire avoids these locations because the veins are not visible. A vampire needs to see the vein for blood extraction, usually done with a syringe or from cutting with a razor-sharp object, such as a scalpel or a razor blade.