The following reasons motivate the current design where the user needs to tap in order to start sign-in:
- The Apple Human Interface Guidelines stipulates that authentication should always be a user-initiated process to ensure that  it is not unexpected that the device will scan/authenticate them,  they are ready to authenticate themselves (eg. is facing the camera),  they understand why they are getting authenticated.
- There is alternative sign-in screen functionality that would get missed if sign-in was immediate. Long-pressing a user overrides biometric authentication and forces authentication through the user's Spectre secret, as well as giving access to the options of deleting and resetting a user's Spectre secret. To force a user to log in only to cancel it or log back out in order to access these is subpar.
That said, there is real merit in your idea and I have thought about it. For instance, the Spectre auto-fill extension performs an immediate sign-in if necessary and only one user is activated for auto-fill. That is because in this scenario, the user has explicitly committed to authentication prior to starting the app and there is no ancillary functionality on the sign-in screen.
I hope this sheds some clarity on the underlying design decisions. Feel free to contribute additional thoughts.