In layman's terms: people in England around ~1400/1500 started to change how vowels sounded in almost every word (i.e., /a/ shifted to /ei/, /i/ to /ai/, ..). G

In layman's terms: people in England around ~1400/1500 started to change how vowels sounded in almost every word (i.e., /a/ shifted to /ei/, /i/ to /ai/, ..). Given that written languages often evolve much more slowly than their spoken form, people simply kept writing everything with its middle English spelling (which now had lots of wrongly placed vowels and mute consonants, such as [gh] and the [k] in "knight"), giving birth to the messy English spelling we all use and love. This is also the reason why English speaking kids find spelling so difficult, and spelling bees exist - people are actually writing in an older version of the language they speak.