You can use ROS with Python and Linux. In fact, that's a big chunk of actual ROS-based systems out there, with most of the rest being C++ and Linux.

For simple robots, you can get away with a simple script. But say, you want to attach more sensors, add some visualization, some logging, etc. Pretty soon you'll find yourself writing multithreaded (not great in Python) and/or multiprocess code, some kind of simple GUI to visualize data, some logging code, some way to manage parameters... and before you know it you'll find yourself writing your own little version of ROS (or more generally, robotic middleware). And without the huge ecosystem of ROS packages that might solve all sort of problems for you.

Good points. You also may find yourself wanting to have more than one robot. And then you may want them to talk to each other.

Robots usually consist of many complicated components. ROS provides a unique and consistent method for those components to communicate with each other via an API and a pub/sub method. That way each one of your components can handle it's own logic via whatever software it likes, just so long as you can connect to the ROS api and let the OS handle orchestrating each component. A bit like a conductor in an orchestra.