The issue is that when you make ignorance a valid defense, the optimal strategy is to deliberately turn a blind eye, as it reduces your risk exposure. It further gives refuge for those who can convincingly feign ignorance.

We should make tools readily available and user friendly so it is easier for people to detect CSAM that they have unintentionally interacted with. This both shields the innocent from being falsely accused, and makes it easier to stop bad actors as their activities are detected earlier.