You /can/, but it's not a use case that github pushes or that people who don't know github would think of. With rapidshare et al you go to the site and it has a big obvious "upload and share a file" button right on the front page.
So? It's possible, and people can make use of it. They may not know, but this can be remedied by communicating. It's also quite a lot better than using some one-off file host such as rapidshare. See Ixiaus' post for more information.
Defaults are important, and what's optimized for one use case isn't optimized for another. An opinionated site that just provides the simplest possible interface for uploading and downloading files is a valuable thing.
(Plus I suspect that if a lot of people started hosting multi-megabyte binaries on github, their policies would change pretty quickly)
There are already many, many mulit-mega binaries hosted on github. It is what the releases feature is for.
But yes, the most simplest site is the best for the most simplest people. However we're talking about people who spent a lot of time into creating their mod/whatever here. They can spend a minute or two more to figure out distribution.
