If you look in RouteServiceProvider.php, you'll see that all routes defined in the api.php-file will use the apimiddleware-group and not web.
This will skip session-handling so you are not logged in when hitting those routes. If you only use the api.php for "internal api-calls", you can simply update this to ->middleware('web').
You find the definitions of the middleware-groups in Http/Kernel.php.
I'm guessing the purpose is to speed up api-routes, and also enable for token-based authentication instead of session-based.
Edit: And if you indent to use Laravel Passport, make sure you provide the access token in you api-calls.