They are and they aren't. Both parties have national bodies that vote on a platform which all candidates are ostensibly meant to support. It's not as rigorous as parties in a multi-party parliamentary system, but it's certainly not true that they are only labels.

It may be narrative, but it's also a major component of many people's worldview -- especially of late, as the polarization has increased. The strength of one's bond to that worldview and the number of people who feel strongly both feed back into the party's platform, as those people are the primary voters (or "base").