I am envisioning two reasons not to have integrity check for the user plane data:
User plane traffic is not generated at eNodeB but RRC message is built at RRC layer. In this case, It is sender responsibility to maintain the integrity of RRC message and receiver responsibility to cross check the integrity.
As compare to RRC messages, huge number of IP packets exchanges over the air. So adding integrity on top of IP packet will be overhead and it will result waste of radio resources.
Say e.g DRB 3,DRB4 (ENB to UE )can we combine and send data into an single EPS bearer (ENB to SGW to PGW) ?