Why is husband's death certificate needed to remove his as a beneficiary?

My husband was listed as the beneficiary on his brother's life insurance policy. He is now deceased, and my brother-in-law wants to change it and make me his beneficiary. The insurance company wants a copy of my husbands death certificate and I don't understand why. I thought you could change your beneficiary to whomever you chose. It's just a minimum policy with enough to cover funeral expenses in the event of his death. He has no other assets and my kids and I are his only relatives.