Accent Marks in English

Accent marks, or more properly, diacritics are not totally absent in English. They are just devilishly uncommon. And the few diacritics I am aware of typically appear in foreign borrowings, such as façade, borrowed from French, or saké, from Japanese. There is also the diaeresis or umlaut, which is used to indicate that the vowels in an apparent diphthong are to be pronounced separately, as in naïve and Noël. This doesn't change accent in quite the same way as you might be familiar with in Spanish.

Other languages with limited use of accents includes German and Dutch, which are, like English, Germanic tongues. So perhaps this characteristic is of the language group, rather than English in particular.

As to "why" I have no idea.

I'm sorry, I'm just going to plop what I know here in simple words because I'm not very good with the fancy terms. Spanish uses accents primarily to find stress in the word (besides ü such as "pingüino" to change the sound of U), while Germanic languages (and even with the French É) such as German and Swedish with ö, ü, and ä actually change the pronounciation of the word. I feel as though people just realized that they borrowed from too many languages and didn't do anything about it for so long (even Shakespeare didn't follow pronunciation rules, really) so they kind of just gave up. It does seem weird though, seeing as all of English's neighbors use some type of accenting in their letters and English basically only uses accenting for a very select few words taken from France..

As far as I can understand, Spanish uses an accute accent mark (á, é ,í, ó, ú), a tilde (ñ) or diaeresis (gü) to indicate a phonetical particularity of a letter within a word; while English uses letter group configurations. Thus, there are not the same phonemes in «off» than in «of», in «late» than in «latte», just to mention some simple examples.