This doesn't apply to goddesses, of course. Who does it apply to, though? Everybody else?
Note that the 'you' in this statement is almost always a woman. Does this imply that men can have it all or are they somehow more realistic in their desires? Here it helps to remember that 'you can't have it all' is normally used to advise women who try to combine having children with having a career, so one interpretation of the truism is that women can't have both successful children and a successful career. Men clearly can. Another interpretation is that it is impossible to be a full-time parent and a full-time worker. This could be true if 'full-time' means 24/7, but then nobody can be full-time anything based on this definition.
The 'can't' means that the 'you' in the statement is unable to 'have it all'. Is this because it is literally impossible (as in 24/7 parenting and full-time work) or because the society is trying to make it impossible (as in the difficulty of combining work and family without access to good, reasonably priced daycare and career paths which allow some part-time work and leaves of absence without terminating the path)? To test these explanations, mentally substitute 'shouldn't' for 'can't' and note if the real meaning changes. If it doesn't, the second explanation applies.
And what is the 'all' you can't have? It might range from being in two places at the same time to having both children and a career during one lifetime. So whether 'you can't have it all' is a useful reminder of life's realities or a lie depends on its exact meaning. But note that in some sense you might 'need it all': both food and water, both work and love. Anyone who asks you to choose between these really doesn't deserve 'to have it all.'
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But you, my dear (fictitional?) readers, can have it all! Athena gave me a hand and now you can talk back to at least one minor divinity.