Comment by simonw
The economic value associated with solving this problem right now is enormous. If you think you can do it I would very much encourage you to try!
Every intuition I have from following this space for the last three years is that there is no simple solution waiting to be discovered.
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.