I think this is the best explanation per Occam's Razor. Licensing is often complex and a lot of developers would rather not think about, plus the alternative is that GitHub by default waives certain copyrights when publishing which might get them in trouble.

It'd be nice if more tooling yelled at you for trying to publish something without a license attached. As much as it annoys me about refusing to even generate a working configure script and Makefile without it, autotools enforcing a COPYING file's presence is a great idea.

I think perhaps a lot of it is also simple misunderstanding. People don't realize that publishing something is not the same as licensing it.

Actually, if you upload something publically to github you give them permission to serve it publically, and to allow other people to read it and fork it. It's part of the Github T&Cs

Read it and fork it, but not to do things that any open source license would allow you to do, like put it on a USB stick and give that stick to a friend.