Other languages do sometimes implement this at the library level. Clojure's core.async comes to mind (though there are subtle differences). There's downsides to this approach though.

The data going into each "mailbox" either needs to be immutable, or deep copied to be thread safe. This obviously comes at a cost. Sometimes you just have a huge amount of state that different threads need to work on, and the above solution isn't viable. Erlang has ets/dets to help deal with this. You will notice ets/dets looks nothing like the mailbox/process pattern.

Erlang is great, but it is hardly the "one true way". As with most things, tradeoffs are a thing, and usually the right solution comes down to "it depends".