Kinda expected the SSH key argument. The difference is the average user group.

The average dude with a SSH key that's used for more than their RPi knows a bit about security, encryption and opsec. They would have a passphrase and/or hardening mechanisms for their system and network in place. They know their risks and potential attack vectors.

The average dude who downloads a desktop app for a messenger that advertises to be secure and E2EE encrypted probably won't assume that any process might just wire tap their whole "encrypted" communications.

Let's not forget that the threat model has changed by a lot in the last years, and a lot of effort went into providing additional security measures and best practices. Using a secure credential store, additional encryption and not storing plaintext secrets are a few simple ones of those. And sure, on Linux the SSH key is still a plaintext file. But it's a deliberate decision of you to keep it as plaintext. You can at least encrypt with a passphrase. You can use the actual working file permission model of Linux and SSH will refuse to use your key with loose permissions. You would do the same on Windows and Mac and use a credential store and an agent to securely store and use your keys.

Just because your SSH key is a plaintext file and the presumption of a secure home dir, you still wouldn't do a ~/passwords.txt.