Specs also evolve over time. There's no ‘end goal’ because requirements are always changing.
Specs are traditionally more forward-looking only because, by removing a lot of the implementation details that are required to write code, the specification can be written to be much broader in scope than code in an equivalent time period. But periodically we invent software that lets us automatically fill in more details of the software that now don't need to be specified by humans, and a level of specification that was previously ‘spec’ turns into ‘code’.
Specs are traditionally more forward-looking only because, by removing a lot of the implementation details that are required to write code, the specification can be written to be much broader in scope than code in an equivalent time period. But periodically we invent software that lets us automatically fill in more details of the software that now don't need to be specified by humans, and a level of specification that was previously ‘spec’ turns into ‘code’.