Why are vowels pronounced differently in English?

A2.1: The English vowels and their pronunciation

EnglishGermanExplanationExamplesaÄopen, short ä, as in casualcat (cat), flat (apartment), tax (tax)aä:open, long ä, like in a sleevehave, can, manaaopen, long a, as if in an armtask, fathereeopen, short e, as in essenlevel, necke–an e is inaudible at the end of a wordhave, giveiishort i, how fit, knowlive, give, mixiei, ailike buckets, ice cream, orphanslife, high, flightOOopen, short o, as in block, wool, headblock, mock, MondayOO:open, long o, like tomorrowdomain, forget, roseuaopen, short a, like hand,cut (cut), mud (mud), unfair (unfair), muggle (non-wizard in H. Potter)wallujulike youthhuman (human)uulike in balldude (dude, dude)uishort i, as in lipbusy