For English, this is because the Great Vowel Shift occurred after the orthography got established and because English has acquired a large number of loanwords at different times, retaining their original spelling at varying levels.
fɔːɹ ˈɪŋɡlɪʃ, ðɪs ɪz bɪkˈʌz ðə ɡɹˈeɪt vˈaʊəl ʃˈɪft əkˈɜːd ˈæftɚ ðɪ ɔːɹθˈɑːɡɹəfi ɡɑːt ɪstˈæblɪʃt ænd bɪkˈʌz ˈɪŋɡlɪʃ hɐz ɐkwˈaɪɚd ɐ lˈɑːɹdʒ nˈʌmbɚɹ ʌv lˈoʊnwɜːdz æt dˈɪfɹənt tˈaɪmz, ɹᵻtˈeɪnɪŋ ðɛɹ ɚɹˈɪdʒɪnəl spˈɛlɪŋ æt vˈɛɹiɪŋ lˈɛvəlz.