Because there's a deadline on initiating a claim, even if you lack all the documents to complete the claim. There's a possibility that if you wait until you have all of the needed documents to file the claim, you will miss the deadline for initiating the claim.

There's a different, more generous deadline for perfecting the claim.

If you initiate the claim now and submit the documentation currently available to you, Chase's insurer will respond and tell you exactly what else you must submit to perfect the claim.