Many people leave items in their vehicles on a continual basis, such as golf clubs, CDs, eyeglasses, etc. If your vehicle is broken into and these types of items are stolen, they are not covered by your auto insurance. Since these items are not a part of your vehicle, they are covered by your property insurance.
Auto insurance covers items that are part of your vehicle. For some things, like special equipment that was installed after purchase, there are limits on what your insurance will pay for loss or damage.