Why does FAA not include certain SIDs and STARs in its CIFP file, even though they are regularly used in flight planning?
Yes it is possible and it is mentioned in the accompanying CIFP Readme pdf. Specifically, it states that


  Procedure types that are not yet included in the CIFP: ILS CAT II, ILS CAT III, ILS PRM,
  Converging ILS and Visual procedures are not included in the CIFP.
  
  Not-In-CIFP Spreadsheet: ILS (Category 1 only), LOC, SDF, LDA, VOR, NDB, GPS, RNAV
  (GPS), RNAV (RNP), GPS Overlays, GPS and RNAV (GPS) helicopter approaches, SIDs and
  STARs that do not appear in the CIFP will be listed in the Not-In-CIFP spreadsheet. Note that only
  FAA-developed procedures are included in the CIFP. Approach types listed above that are developed
  outside the FAA, such as Navy or Air Force procedures, will not appear in this spreadsheet.


In other words, even though you can at least acknowledge their existence by searching for the missing SIDs and STARs in the aforementioned xlsx spreadsheet, you do not have any information about their structure. As for why they are not included I can not tell for sure, but as far as I could realize from my own research, the few procedures not included in the CIFP file belong to the "old" IFPs (conventional, non-RNAV). This might mean something, but in any case it is my own (un)educated guess and by no means anything that can be confirmed from an official source.