Python is not just a single executable. It takes a little more work than one would want. And for what gain? Would you be fine with other software also requiring a custom interpreter to function? It gets cumbersome.
Incredibly. But it seems to be The Ruby Way™…
If you rely on the system-wide Python you'll need to cover different versions and you need to make sure not to use any 3rd party modules.
Ansible ships its own libraries, generally.
Well, it's an agent-less system after all, the idea is to keep the system lean.