Why does AES ciphertext entropy always close to 8?
What you did in your entropy estimation is effectively modeled the process of selecting a random byte from the file, and computed the entropy that would be in that byte? You can do that (and, since you have the exact probability distribution, you could compute the Shannon entropy). However that would also answer your question - such a process would give you an 8 bit value; of course the entropy in that byte can't go beyond 8.

(The comment converted into an answer because, well, it appears to be the answer)