Because their clients want to deploy Linux AI workloads directly on the mainframe to gain lower latencies and higher troughput and ARM has better library support than s390x. Half of the last two mainframe-dedicated Hot Chips presentations were about the Spyre inference accelerator. Every newer mainframe CPU also has a big inference accelerator built-in, which is accessible to ARM binaries at instruction-level latencies (and has dedicated s390x instructions).
Sure, don't disagree, RISC-V has 10Gbit nic, 32GB, multi core machines available now. Software slower to get there but catching up quite quickly. Don't discount that too.