I'd wager the use of "the" is mostly about making the meter work and having the iambic pentameter sound the way he wanted it to. This isn't a data science question. Especially if the data science is blind to meter and to phonetics.
why "the" but not "a"? Same meter. But I agree it's silly that say "the" is what makes Macbeth creepy, and not, you know, the occult theme that permeates it.
"Give me thuh cat toy." (Some ordinary toy.)
"Give me thee cat toy." (The one with special powers.)