Good question.
The cells in multicellular organisms are not capable of surviving on their own (usually - there are exceptions) because they are missing pieces of cellular anatomy or cellular chemistry that are necessary to provide all of the functions of a living organism. They may not be able to ingest food. They may be used to a certain temperature range that is normally supplied by the body of the organism in question. Or they may not live long even in a multicellular organism, and are normally replaced in a multicellular organism's body by new cells.