That's because false/fraudulent death claims are about as old as contracts. For anything month-to-month, if you as the executor or other authorized person have access, you just cancel it and be done with it. For anything where the company was expecting ongoing revenue on a contractual basis, they're going to want proof that the customer is dead before they cancel the service. It also helps fend off people who simply want to make trouble for others by interfering with paid services.
That doesn't get into all the other things you need death certificates for: survivor benefits from Social Security, Medicare, the VA, and pensions; at least one for every financial institution; another for each life insurance policy; possibly another to change health insurance coverage; final tax filings, which may require one each for federal, state, county, and city; probably one for probate; and miscellaneous others that inevitably come up specific to the decedent's specific situation.