Well if you will notice all this framework are abstraction over WSGI or ASGI (these are standards in Python Language interface for web programming). So the situation is not same as PHP.
Moreover if you look at the framework code of many of them you can replace them easily, for example Flask and bottle.py can be completely replaced by your own code if your service or app grows the intended performance. The lowest common denominator in both is WSGI.
Moreover if you look at the framework code of many of them you can replace them easily, for example Flask and bottle.py can be completely replaced by your own code if your service or app grows the intended performance. The lowest common denominator in both is WSGI.