Many compression algorithms are effectively string interning that works on general-purpose binary data and adaptively pick the common substrings that are most repeated and assign them the smallest bit representations. That's why formats like XML and JSON compress so well: all those repeated string keys get stored once and then become sub-byte entries in a lookup table.
Good point! And since we can have sub-byte entries in a lookup table, no wonder why a simplistic string interning solution using pointer-sized entries in a lookup might not work as effectively to reduce memory used.