Suing Github = signing up for a ~decade long incredibly expensive and time-consuming legal battle against one of the richest companies in the world
There may be a slight difference in effort between these two options.
[0] https://techcrunch.com/2012/03/22/microsoft-and-tivo-drop-th...
If a company or person assume they got copyright permission to any work public accessible then they will quickly find out that such assumption is wrong, and that they require explicit permission.
And why should opt-out be a reasonable norm? To be clear, the internet (among many other things) breaks down if every exchange of information is opt-in. Sharing of photographs taken in public places is another example. But the internet basically functions because people share information on an opt-out basis (that may or may not even be respected).