Because it's really, really slow compared to in-language variables? I've not yet ran out of memory because of program variables, and speed shouldn't be an argument for low memory footprint when it's already at best 1/10 of it's performance.

Redis locally is good to do IPC though. You have many scenarios where multi local processes have to exchange data. You have alternatives but Redis works and can scale up.

You can do a mutex in like 10 bytes, how could redis be more memory efficient?

you are using the minority language if that's the case for you.