Why no Grave Accents in Shakespeare Plays?

We recently encountered a question from some Shakespeare-savvy English teachers asking why the plays don’t include the grave accents. The grave accent in English indicates a pronounced “e” where one normally finds a silent “e,” such as in the word “blessèd.”
In his “Introduction to Hamlet” Richard Leed states that “The past tense ending –ed is usually pronounced as in modern English but sometimes Shakespeare takes the advantage of an old-fashioned pronunciation by allowing it to be pronounced as a separate syllable.” Shakespeare did this to attain his most favored pattern—iambic pentameter. (Iambic pentameter is defined as a ten-syllable line with the accent on every other syllable, beginning with the second one.) Our research has shown that most modern editions, however, including the definitive Riverside Shakespeare published in 1974, have dropped using the grave accent to indicate such occurrences (about fifty of them in Hamlet according to Leed).
EMC’s Mirrors & Windows program has referred to Riverside as the main source for Shakespeare’s works and accordingly does not include the grave accents. “There are a number of instances where printed editions of the plays do not consistently reflect the correct number of syllables to be pronounced,” says Leed. “Such cases include the past tense ending ­­–ed, standard contractions, and certain words.”
Out of curiosity, I checked the text of Hamlet that can be downloaded from http://www.gutenberg.org/, but alas, this version did not include the grave accents either. I thought that if I went back as far as some of the earliest American editions I might find them, but when I checked the text of Hamlet in The Dramatic Works of Shakespeare (on Amazon, believe it or not), published by Hilliard, Gray, and Company in Boston in 1836, I could find no grave accents! It appears one might have to go back to earlier editions published in England--does anyone out there know of an edition currently in print that includes grave accents?