For the same reason that "flood" is pronounced the way it is!
More helpfully, I found this somewhere on the internet:
It might be worth bearing in mind that English spelling often records the pronunciation 600 years ago. And at that time, blood, flood, food and book would have all been pronounced with the same vowel. However, pronunciations have changed since then, so the spellings are a little unhelpful for learners of English today!
By Shakespeare's time, all of these words had the 'oo' vowel as in food (the traditional vowel with the lips rounded, not the fewd pronunciation mentioned by Fish). What happened was that the vowels of blood and flood shortened in the 16th century and had the vowel sound of put. Accents in southern England changed this sound to the one in putt.
Book, look and foot, however, had the shortening occurring later, and the put vowel remained, and did not change to the putt vowel.
And food did not shorten its vowel.
Sounds plausible to me!