Because you'd already have a highly secure base with no centralized app store (Google) built in by default. 

What would make it a "nostr phone" a "nostr phone"? I guess thats the question. I imagine,

- built in nostr onboarding and key creation for new users
- System level  "Sign in" for exsisting users (this store your key locally in some kind of system-wide Amber/local nsec bunker type funtionality)
- Nostr-centric app store like @Zapstore to install your apps (you already have your key stored locally so you wouldn't even have to sign in to see your web of trust)
- Contacts Intergrared with nostr, similar to Mutiny
- A selection of daily use apps added out the box, ideally most that rely or intergrate with nostr


Of course, you'll always be able to just install the apps, but I guess this would be an expereience taylored to work really well out of the box, using as much cutting edge nostr stuff as possible at the system level.