It's also different from Redis because Redis is intended to be ran as a server (it stands for Remote Dictionary Server). This is ran entirely as a process and communicates via IPC; other machines can't reach the database, only the local machine. This is a big reason for why it's very fast. However, it also means you can't distribute the database across multiple servers.
You could think of it like a very fast NoSQL Sqlite, I guess.
For the same reason you can't do that with MySQL or Postgres or any other database server. Redis intended to be ran as a server listening for TCP connections, and communicating over a TCP/IP network. If you're running it on localhost, the communication should generally be pretty fast, but you're right that overhead will still be incurred.
It would require adding a lot more code to allow for typical interprocess communication.