So they are harder to introspect and review once set.
It’s probably good practice to put non-secret-material in non-sensitive variables.
(Pure speculation, I’ve never used Vercel)
There are cases where I want env variables to be considered non-secure and fine to be read later, I have one in a current project that defines the email address used as the From address for automated emails for example.
In my opinion the lack of security should be opt-in rather than opt-out though. Meaning it should be considered secure by default with an option to make it readable.