It's because the term itself got overapplied by people critical of LLMs -- they dismissed all LLM-assisted coding as "vibe coding" because they were prejudiced against LLMs.

Then lots of people were introduced to the term "vibe coding" in these conversations, and so naturally took it as a synonym for using LLMs for coding assistance even when reading the code and writing tests and such.

Also because vibe coding just sounds cool.