Why can’t I just give the store receipt from purchasing my jewelry to the insurance company instead of paying for an appraisal?

If you have a loss, the burden of proof is on you to show the insurance company how much you have been damaged.  Many jewelry store receipts use generic descriptions without even mentioning the total carat weight of the gemstones in a piece of jewelry, let alone the quality of the gemstones.  Many insurance companies will not write you a check for what you paid for a piece of jewelry or even what it is appraised for.  They want you to go to their jeweler to get a replacement.  In order to get a replacement equal to what was lost, you need a detailed description of the jewelry including identification and grading of gemstones.