How to get access to administrative accounts on Android?
OEMs as well as the Android foundation itself try to strictly not have such an Admin user available to the user (security reasons)
and thus not provide such a user, which in general is true for most OEMs, but some set up a special system user with extended permissions for some hardware/software parts used by their own apps (they might have a user+password combo but they are well hidden and usually useless).

As for security reasons: Think about it, someone, who knows the admin's credentials, writes a malicious application that steals passwords, credit card information and injects ads everywhere, maybe even lock down your phone and wants you to pay to unlock it.
Then you are in big trouble. To prevent this, Android features SELinux (security enhanced linux) which prevents system wide modifications on specific settings (every user/app has its own namespace). System applications also run in their own namespace, but have system permissions to allow further modifications (non-malicious ones).
Rooting allows you to run specific commands as the SuperUser who has even more permission than the system user, but always allow the risk of malicious modifications.


  Because Android wants to establish more as a business OS and advertises a secure OS, access to the system user is very restricted and the SuperUser is completely removed/stubbed and useless. In other words: Security > user permissions.


Linux also allows the removal of the system/root user, btw.