Okay, I figured out why this is happening, after reading this article and this answer.
If you have access to a WiFi network because at one point or another you or someone else explicitly shared the password with you using the "Share Password" feature on iOS 11+ and macOS High Sierra+, then you never actually received the true plaintext password and so it is not saved in your keychain.
Instead, you get a kind of alphanumeric hash that was based on that password. Your device can use that hash to connect to the WiFi, but it's not feasible to reconstruct the original password from it.
I am seeing these alphanumeric hashes for certain WiFi networks because I must have entered the true password on iOS and then explicitly shared it to my Mac (instead of relying on iCloud Keychain sync, to sync the true passwords across devices).
since that hash is all that's saved in my keychain, I have no way to recover the true password
whenever you use the "Share Password" feature to share a WiFi password to nearby devices, you're not actually sharing the password but only access to the network.