Your IP address is needed so that responses are sent back to you.
Other explanations: https://stackoverflow.com/questions/60233953/why-does-an-ip-header-need-to-have-source-address
What's probably important in privacy policies is explicitly stating that IP addresses are not retained, not stored, not collected. If a service is described as collecting IP packets, then that includes collecting IP addresses. But RPC providers probably don't describe themselves as collecting IP packets, and so describing themselves as collecting IP addresses sounds like they retrieve the IP addresses from the IP packets, discard the rest of the packet, and retain/store the IP addresses.