Companies are made of people. Those whose wealth and wellbeing depend on Github being the monopoly may be interested in destroying the competition before it grows into a concern.
It would be really stupid for Microsoft, the country's richest company, to go after a tiny competitor via extralegal means. Absolutely would make no sense. The liability they would incur would be astronomical
Not to mention the free publicity for their competitor if they're caught.
"We're so good, Microsoft was scared of us"