Why does android init not die? As I read from the book Embedded Android (Karim Yaghmour), android init does not die. However, Linux init systems usually die. Why doesn't the android init?

And also, why does android not use / need systemd?
Init is one of the first processes that is loaded when the Bootloader starts in Unix.
So, technically init is the first process that starts and all other processes are either children or the grandchildren of init. 
Now, when a process say Ystarts execution, it is usually forked from its parent process, say X when X dies(finished executing) the process Y now has no parent, and it becomes an Orphan Process at this time init takes over the ownership of the process and becomes the parent of Y.
Like this, there are so many processes running at a given instant in the OS, everything has to be taken care by init.
Since Android being a flavor of Linux, and Linux being a derivative of Unix, it is for the same reason that init doesn't die,unitll the system shuts down.