Want to clarify a bit. There is not RRC-integrity and RRC-ciphering. Integrity and Ciphering are two processes.
Integrity refers to maintaining and assuring the accuracy and consistency of data over its entire life-cycle.
Ciphering: put a message into secret writing (put (encode).
Now coming to your question why it is like this: In layman It is just shifted the work from one protocol to other protocol. Dividend rule :). The main purpose is to get the work done. Who is doing that's not compulsion. PDCP layer has to be there to do the ROHC so It can have more work. No need to make the lengthy code for RRC. That's what I think. Rest they might have some technical aspects also. Might other will emphasize on it.