It is because there is a difference between basic training and operational training.
If one takes ICAO an an example, Annex 1 deals with basic training and Annex 6 deals with operational. This translated into State regulations leads to the parsing of the two tasks and, for Europe, is the difference between JAR-FCL and JAR-OPS 3 Subpart N. If it were any other way, basic training would take forever and cost a fortune.
Another consideration is operational recency; some skills - long line, SAR, HEMS - have a limited currency.
Jim