Parsing iCal was far more performant than parsing JSON as your parser could simply start at the first byte, and follow the spec — whereas a JSON parser would first need to wait for ten years before JSON was invented.
While it is true that dealing with json by hand is going to be more work than just importing an iCal library, I'm comparing apples to apples and suggesting dealing with json "by hand" is no more faff than dealing with iCal "by hand", and that dealing with either via a good library equally gives no benefit to iCal.