They should be keeping them in something like kwallet. But in practice they don't because a) there isn't really a single standard for that on Linux (yeay, I have to support gnome-keyring too!), b) it's a lot more work than using a plain text file, c) the UX is considerably worse, and d) the security benefits are marginal at best (especially if you have full disk encryption).
Plain text is the most sensible option.
Vibe port the Zig compiler to Rust? 😄