Strength of Calling Range vs Opening Range

Over and over again in various books/podcasts/videos I'm seeing people say that post-flop, the open raiser should be considered to have the stronger range than the caller, and so aggression from the raiser should be given more respect than the caller. I would have thought the opposite is true? Say is the raiser is opening 30% of hands, then knowing this wouldn't the caller only call with hands that are at least as good as the average hand in the openers range, i.e. the top 15% of hands. So the caller should have the stronger range?

I know that callers should actually call a bit wider than this, because either they have position or are in the blinds and are getting a discount to call, and that premium hands can be excluded from the caller's range as they would be 3bets rather than calls. But I'm not seeing how these can have such a big effect as to give the caller the weaker range?

Or put more simply - if the caller is going to have the weaker range after calling, then why call?