LTE network provides IP network connectivity through which UE can communicate to other server and devices in IP network. Similar to your laptop or computer when it sends any thing towards other servers it has to form an IP packet that's why it needs source IP address while originating IP packets.
Rather than introducing any other mechanism at UE, standards bodies preferred to use IP stack at UE side.
Another point is UE is either in WiFi or LTE network, from user point to view he wants to access internet. It can form IP packets to send to other servers/devices in network that's why it needs IP address in advance, which is provided as part of attach procedure.
Introducing a proprietary stack and then mapping operator specific UE identities to IP address while sending out packet from LTE network to external world, will cost more and unacceptable technology in this era where each operator is trying to reduce CAPEX/OPEX and mobile manufacturer wants to keep mobile prices less.
