Yes, and the result is undoubtably trash. I have yet to see a single vibe-coded app or reasonably large/complex snippet which isn't either 1) almost an exact reproduction of a popular library, tutorial, etc. or 2) complete and utter trash.

So my question was, given that this is not a very hard thing to build properly, why not properly.

The choice with this kind of question is almost never between "do it properly or do it faster with LLMs".

It's between "do it with LLMs or don't do it at all" - because most people don't have the time to take on an ambitious project like implementing a new programming language just for fun.