From a recent report by Social Security's Office of Inspector General (OIG):
Each month, the VA [Department of Veterans Affairs] furnishes SSA [Social Security Administration] with automated death records. Before SSA terminates benefit payments or records death information on the Numident [a major SSA database], SSA employees must independently verify the VA death information. In April 2016, we obtained data from VA identifying approximately 17 million deceased individuals. We matched the VA data against SSA’s payment records to identify potentially deceased beneficiaries in current payment status. ...
SSA issued payments to 3,925 beneficiaries who had dates of death in VA’s records. Our audit results indicated that at least 11 percent of these beneficiaries were alive, and death information in VA’s records was erroneous. However, our audit results also indicated that at least 19 percent of these beneficiaries were deceased , and death information in VA’s records was accurate....
You can see Social Security's problem. They're receiving a lot of bad information from VA. Mistakenly declaring someone dead when they're not is a very bad thing. Not only are their Social Security benefits ceased, their bank accounts and credit cards are frozen and they can no longer receive medical care. Social Security has to independently confirm that the person has died. That can take a little time and mistakes will be made. In the end, even though Social Security received 17 million death reports from VA, OIG could only identify 19 cases where individuals had died but benefit payments were continuing. Nineteen out of 17 million is actually a pretty good record. That's an error rate of 0.00011% if my math is correct. Of course, the right wing Washington Times accuses Social Security of incompetence.