Android is not a single user system. Every app, every service basically everything gets its own user.
Applications have different user IDs and different SELinux contexts.
Android security is tight
Does Android use Udisks? I assumed it did not, due to the difference in architecture over most traditional GNU/Linux desktop systems
I haven't actually looked at the numbers, but I strongly suspect that it's true that the overwhelming majority of single-user Linux devices out there are Android devices. If that's true, then it's my understanding that Android does bother to fairly properly sandbox programs from each other... so an escalation to root would actually be a significant gain in access.