Senior engineers either already know exactly where the changes need to be made and can suggest what to do. They probably know the pitfalls, have established patterns, architectures and designs in their head. Juniors on the other hand don't have that, so they go with whatever. Nowadays a lot of them also "ask ChatGPT about its opinion on architecture" when told to refactor (a real quote from real junior/mid engineers), leading to either them using whatever sloppypasta they get provided.
Senior devs earned their experience of what is good/bad through writing code, understanding how hard and annoying it is to make a change, then reworking those parts or making them better the next time. The feedback loop was impactful beacause it was based on that code and them working with that code, so they knew exactly what the annoying parts are.
Vibe-coding juniors do not know that, their conversation context knows that. Once things get buggy and changes are hard, they will fill up their context with tries/retries until it works, leading to their feedback loop being trained on prompts and coding tools, not code itself.
Even if they read the outputted code, they have no experience using it so they are not aware of the issues - i.e. something would be better being a typed state, but they don't really use it so they will not care, as they do not have to handle the edge cases, they will not understand the DX from an IDE, they will not build a full mental model of how it works, just a shallow one.
This leads to insane inefficiencies - wasting 50 prompt cycles instead of 10, not understanding cross-codebase patterns, lack of learning transfer from codebase to codebase, etc.
With a minor understanding of state modeling and architecture, an vibe-coding junior can be made 100x more efficient, but due to the vibe-coding itself, they will probably never learn state modeling and architecture, learn to refactor or properly manipulate abstractions, leading to an eternal cycle of LLM-driven sloppypasta code, trained on millions of terrible github repositories, old outdated API's and stack overflow answers.