Those customers don't use mainframe, they use POWER. There's been a handful of POWER supercomputers in the past decade built for essentially that reason.
POWER is not uncommon in HPC, but IBMi (which is very enterprisey) is also based on POWER. You won't find IBM mainframes in HPC, but that's because HPC is not as sensitive for latency and reliably than online transaction processing, and, with mainframes, you are paying for that, not for TFLOPS.
Yeah the architectures focus on different things. The huge caches on the z series chips are designed primarily around the kind of latency sensitive workloads more common in finance than the massive floating point throughput often needed for big scientific computing.
