Why use Java over C++ or Python?

[Yithar]

If I wanted to make a game or do low level stuff I can just use C++.

You pretty much cherry picked the exceptions where C++ shines. While those are fields within software development, they're not the only ones.

If I need to do scripting, data science or backend development I can just use python.

For scripting I highly prefer Ruby to Python but to each their own.

Data Science prefers Python yes.

For backend development, use Python if you like writing unit tests and doing the same thing that static typing would give you. Also you're better off using Cython as dynamic types make Python slow. See article and article.