Because they're zero-width. If you can't see them when you print your data, it's a machine-only separator, which makes it a bad separator for data that humans need to look at and work with.

(Because CSV is a terrible data exchange format in terms of information per byte. But that makes sense, because it's an intentionally human readable data exchange format, not a machine format)

Hence https://github.com/SixArm/usv/tree/main/doc/faq#why-choose-u...