Most of the time when the tower is closed, the tower freq becomes the CTAF. There may also be a UNICOM or MULTICOM as well. The final word can be found in the current APFD.
I have found many examples of this, and was wondering the same thing. Also, I am finding many Nav-aids are missing as well. I like to Cross Countries, and file and fly IFR with an instrument approach at the end. Kind of hard to do without the appropriate ground facilities available. Any ideas?