If somebody wanted to package a modern Python application as Debian package they’d have to make a Debian package that contains several Python packages maybe as a venv or executable wheel, it is a solvable problem but a bit like comparing tic tac toe to 12d chess with supersymmetry in terms of the ambition of Linux distros if not the difficulty.

If you installed python packages as debs without any isolation you'd never be able to install two applications that depended on different versions of the same Python package.

The best thing about Java is that it is so xenophobic that nobody expects to have a working Java runtime installed with the OS so you never have to deal with a broken runtime installed with the OS. JAR files are remarkably self-sufficient so "write once one anywhere" is way closer to the truth than it is on "DLL Hell" platforms like Windows and Linux.