It's the "Ship of Theseus" paradox [1]. Choosing a semantic key means mixing identity and attribute, while a synthetic key solves by assuming "constitution is not identity".
Since a digital system is a model of the world, a synthetic key allows the system to address objects in this internal model without assuming a particular interpretation of identity in the real world. E.g., it's often the case that you do need to have two "customer" entries in your system that represent the same physical "person" in the world, and this is ok because the concept of "customer" is useful and sufficient for your model, and the physical person isn't.
More often than not, people get caught on this trap in relational databases and object oriented modelling. This can be seen in books and lectures that use Customer-Order-Product relations to teach databases, or Car-Engine to teach OO.