That's how encryption keys are typically stored.

If someone has enough access to your system that they can get to your stored encryption keys, you have much bigger problems.

At-rest encryption is not something that Signal Desktop is currently trying to provide or has ever claimed to provide. Full-disk encryption can be enabled at the OS level on most desktop platforms.

Next in the news: cloud infrastructure engineer had ssh keys in plain text on their MacBook exposing 99999 servers! Everything you ever stored on the internet is potentially compromised!