They are not as rare as originally thought - significant densities of ice crystals near (tropical) convective storms appear to be relatively common.
The problem is that flying through these ice crystals does not always seem to have an effect.
Agreed that they are not as rare as once thought (and the frequency appears to be increasing - perhaps due to climate change/global warming). But given the thousands of flights that occur daily and the handful of actual events, it's still pretty rare.
There was discussion several years ago of using an engine company flying test bed, with a highly instrumented engine (including things like internal cameras to record ice formation) but it was ultimately rejected as too expensive and impractical due to the difficulty of finding the right conditions.
The prospect of doing fight tests in ice crystals using an FTB is being revisited in light of the recent GEnx events.