Because unlike many countries we donu2019t have a national ID. Efforts to institute one have failed for a variety of reasons, but mainly those having to do with privacy and government surveillance. It would also likely be unconstitutional.Such an identifying number is useful for sharing medical data between labs, practices and hospitals. It is also necessary for bill collecting. Without a national ID the social security number is used as an alternative since nearly everyone needs to be registered with the Social Security Administration.