Just because it can doesn't mean it's tolerable to have one core of their CPU eaten up just working on AES all day. Plus if it wasn't task set to a CPU you'd see massive latency hits on disk access even the CPU could keep up. The kernel still has to schedule the task and load balance.
EDIT: in fact I just looked up some benchmarks for Ryzen and it can do 3GB/s per core. So that should be enough