If your only communication mechanism is actors, then you're right: several process would be fine. But I think that almost any software built with actors requires some shared memory as well. Even Erlang has ETS. In the demo, we use an in-memory spatial database. It's this shared data store that's hard to get right with several processors, especially if it is supposed to help with parallelization and scheduling.

BTW, Quasar also has distributed actors.