Because there isn't really a such thing as a "build" for a Python web app, it's a dynamic language. It's not like Java where you'd have to recompile the whole app if you packaged your config inside the compiled jar.

In Django apps, there's an env var called DJANGO_SETTINGS_MODULE that points to one of multiple settings.py files, and you change that var depending on which environment you're working in. Then typically you also would want to store individual variables that need to be secure (stuff like any secret keys and database credentials) in env vars, but the overall structure of your config is just a python dict in settings.py.

For reasons that Yegge touches on in the blog post I linked above, you really want a tree structure for configuration of any complexity, and env vars don't provide that.