Hi! I haven't found the UserVoice of Outlook, so I'm posting it here:
There should be an option on Outlook for mobile to ask for credentials when opening the app.
Reason: If someone steals your phone while it's unlocked (yeah, I live in Brazil), that thief will be able to open your mail app and start asking to reset services passwords and sometimes some bank accounts like Nubank.
OneDrive app has that option already.