[Python] Why asyncio.get_event_loop method checks if the current thread is the main thread?

Why get_event_loop method in asyncio (source) is checking if the current thread is the main thread (see my comment in the snippet below)?

def get_event_loop(self):
"""Get the event loop.

This may be None or an instance of EventLoop.
"""
if (self._local._loop is None and
not self._local._set_called and
isinstance(threading.current_thread(), threading._MainThread)): # <- I mean this thing here
self.set_event_loop(self.new_event_loop())
if self._local._loop is None:
raise RuntimeError('There is no current event loop in thread %r.'
% threading.current_thread().name)
return self._local._loop

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