Expo api route

[LaCapitainerie]

It's been a week since i'm looking for repos with Expo and Better-Auth to speed up building mobile apps by using all the plugins Better-Auth offer, but in this quest i saw everywhere people using clerk and supabase instead of better-auth, AND people using 2 services one Expo for the mobile app, and another with NextJs for the server, but i saw by reading the Expo doc that Expo also have Server api route included by writing file as name+api.ts,

So now i'm wondering why people are using 2 separate services to build their app if the main goal of native is to build the 2 service in a single one, as with a trpc api route and a better-auth api route it could theoretically run only on one Expo app ?

I'm guessing i should be missing something like NextJs outperform Expo in performance or NextJs have much more library to make server code than Expo or somthing like that, but i'm curious to know the reason