Because almost every programmer these days has learned by a route that relies on incorporating code on trust. E.g. using someone else's compiler.
> I use code from LLMs on most work days ... not because I trust it, but because I understand it and tested it.
The snag there is that one can rarely verify test coverage is sufficient.
> the same as code from anyone, including from myself six months ago.
But it is not the same. ChatGPT delivers completely untested code. Not even syntax-checked in my case. Would you accept such code from any human?
> Trust doesn't enter into it. Clear and workable solutions do
Plenty of clear and workable solutions are discovered unsafe. I suspect that's much more likely when the code comes from a source such as ChatGPT that delivers code in a completely untested state.