A large part of the problem is that the conditions are nearly impossible to re-create on the ground (among other things they can't generate ice crystals that small).
Is that an issue to do with generating the sheer number of crystals required? It's quite easy to generate these small crystals in the lab, but I'm unsure how it'd scale up to the number density required for flow through an engine.
And even in the convective weather systems that can result in ICI issues, the actual conditions that adversely affect the engines are rare.
This is debatable, in my view
These HAIC seem to form much more frequently than was originally thought, but mainly in areas where there are relatively few flights (tropical Africa and equatorial Atlantic from my work, not sure about the other size of the globe). Something that interests me is why North America, despite having many large convective storms, doesn't seem to be affected by this ice so much - we rarely see it there. I guess the other important question is just how much of this stuff we need in the air before it starts causing problems.