If swearing is inappropriate and impolite, then why is it considered masculine?

It's not. Profanity is the language of emotion and conveys little else. Anyone who uses a lot of profanity is being very emotional, which is not a masculine concept. In the movie Extraction with Chris Hemsworth, near the beginning the Aussie fellow says to the other guy, "Stop swearing all the time. It makes you sound stupid."

Ever read a history book? It isn't exactly like men have put a lot of effort into being considerate and kind