Actually, they have.
"Definition of LITERALLY
1
: in a literal sense or manner : actually <took the remark literally> <was literally insane>
2
: in effect : virtually <will literally turn the world upside down to combat cruelty or injustice — Norman Cousins>
Usage Discussion of LITERALLY
Since some people take sense 2 to be the opposite of sense 1, it has been frequently criticized as a misuse. Instead, the use is pure hyperbole intended to gain emphasis, but it often appears in contexts where no additional emphasis is necessary."
http://www.merriam-webster.com/dictionary/literally