Why is there no JRE for android? I understand that android does not use a JRE and Google has build a platform only for android. But an android device is not really different from e.g a Raspberry PI. So why is it not possible to just install Oracles JVM on a rooted android device?
Why is it not possible to just install Oracles JVM on a rooted android device?



You simply don't just install something on Android. Installing apps requires a bunch of stuff.
Also: You can't just install ordinary applications, frameworks etc like on a PC (there are apps that allow this in their own container).
There is an ongoing dispute between Oracle (Java) and Google (Android) about Java APIs and code. (https://en.wikipedia.org/wiki/Oracle_America,_Inc._v._Google,_Inc.)
You are already running a Java virtual machine that is integrated into the system (DalvikVM, Android Runtime ART) and is able to run basically all non-proprietary Java code (no sun package).
If you really want to run Java code, make a simple one button app that calls the code you want.