I am just wondering why vanity URLs are not standard on all Nostr apps, when my metadata linking my public key to a username is published to all relays and therefore available to all apps (or is it?)?

Why isn't appname.com/username the de-facto standard?

For example, I am https://primal.net/avichand on the Primal app, but not so on the Nostr.band app or the Coracle app.

Why is this so?