Literally how many times have we heard someone say “literally” when they mean no such thing? I heard Trump say something the other day like that and when I find the quote I will post it here. But it is ubiquitous.
A similar usage pattern is “unique”. He is a unique person. Well, aren’t we all? But if you mean he is different from everyone you have ever known, it might work. But if you just mean he does something well or has some interesting features to his personality, you are transgressing the meaning of the word. To say someone has a unique personality is not the greatest sin since people can have a combination of traits that might be hard to match, or even a get-up they are wearing, but to say Joe is uniquely capable of repairing toilets, you are on shaky ground.
Why does this happen? Linguistically, it is possible people just do not know the exact meaning of the words and so misinterpret from contexts like, “I was literally shaking with fear” where literally means literally but can be interpreted as “really” and “unique” might be interpreted as “special”. Such processes are well covered in standard works on language and linguistics. The other process is weakening, where a meaning gets diluted by overuse.
Pedagogically we can either just let it go as the normal erosion of meaning amidst various factors like overuse, obsolescence, interference from other dialects or languages, salience in use (a much bigger factor now with electronic communications, starting with the radio), or we can get students to read literature in which these words are used according to the preferred (by us) definitions and offer students drills in using the words (about as effective as antibiotics on a virus).