Whether you're an English native speaker or just learning the language, you must've noticed that English spelling is absolutely mad at times. Why doesn't meat rhyme with great, for example?
One reason for this is something known as the Great Vowel Shift, which took place between the mid-14th century until the end of the 15th century.
If you spoke English in the 1300s, bite sounded like beat does today. The word meet sounded like mart, boot like boat, and boat sounded a bit like bought. In fact, during the Great Vowel Shift, every long vowel in Middle English changed its pronunciation.
Pronunciation tends to change over time in most languages without causing too many problems. However, around the time of the Great Vowel Shift, the printing press had made its way to England and was in the process of standardising English spelling.
Some English spelling follows how words were pronounced in Old and Middle English and wasn't really changed to keep up with Modern English. Though some, such as room, no longer uses its Middle English spelling, roum.
At a time when people finally decided how words should be spelled, the language underwent a significant pronunciation change.
Think of it as getting your passport photo taken and then immediately shaving off all your hair (or growing it, if you're bald)!