Because it's a joke. It's just not Python. Not half of the Python libs work on those things. You don't have serious introspection, most of the stdlib is missing

Because it's a joke. It's just not Python. Not half of the Python libs work on those things. You don't have serious introspection, most of the stdlib is missing, the stack trace doesn't work as expected, you can forget about async / await and yield from, etc. To run Python in the browser correctly, you need the entire 3Mo Python VM. Transpiling is not enough because Python semantics are complicated as it is very, very rich. Comparatively, js is incredibly basic.