I don’t understand this decision, either.
For one, many aircraft in the sim pre-date ICAO, were built in such limited numbers, don’t exist in any airworthy capacity, etc. that they do not even have an actual ICAO model designation.
Like the multitude of “made up” ICAO-named US airports in the sim, we now have made up ICAO model designators for aircraft like the Dornier Do X or the Hughes H4 Hercules, etc., and those are used for the sort order?