I mean if the wife says to her husband: The traffic light is green. Then this may count as an instruction to get going. But usually declarative sentences aren't interpreted as instructions. And we are perfectly able to not interpret even text with imperative sentences (inside quotes or in films etc) as an instruction to _us._ I don't see why an LLM couldn't learn to likewise not execute explicit instructions inside quotes. It should be doable with SFT or RLHF.
Perhaps prompt injection attacks currently occur (or appear to occur) so rarely that the economic value of fixing it is actually judged to be low, and little developer priority is given to tackle the problem.