The short, perhaps jokey, but absolutely true and definitely the best answer is 'just because'.
Some people refer to English spelling as a pseudo-historical and anti-educational abomination, while some describe it as the world’s most awesome mess. There are reasons and causes for this. Also as I often say 'there are more exceptions to rules than there are rules'. So you'll have to learn the pronunciation of every word by rote.
The answer is going to be extraordinarily long if all the details have to be included, I'll just explain it briefly. There are many reasons that account for the irregularities in spelling and pronunciation. English is a mix of several languages including Old English, French, Latin, Norse, Greek, German, Spanish, Italian etc. I'm going to discuss a few reasons.
Old English Period
Old English was a very different, pretty phonetic and heavily inflected language. There were no silent letters in Old English. For example, the k in the word 'know' is silent in Modern English, it wasn't silent in Old and Middle English. The word 'know' used to be cnāwan in Old English, and it was pronounced how it was spelt i.e. /ˈknɑː.wɑn/. The [k] was dropped because 'Plosive + Nasal' combination was awkward for Modern English speakers. They dropped the [k] from pronunciation but kept it in the spelling.
In Old English, ⟨þ⟩ and ⟨ð⟩ were used interchangeably to represent the th sounds. They were voiced between vowels or two voiced consonants, and voiceless elsewhere. We still see the remnants of this property in half - halves, bath - bathe, close (adj) - close (v). One word has a voiced sound, the corresponding word has voiceless etc.
The gh in night, knight, might etc., wasn't silent but was was pronounced /x/ (voiceless velar fricative).
There were many other strange processes in Old English, for instance, 'i-mutation'. The results were foot-feet, goose-geese, tooth-teeth, mouse-mice etc.
One of the main changes in late Old English was homorganic lengthening, whereby vowels were lengthened before /ld/, /mb/, /nd/, /ŋg/ etc., so you see that child, mind, tomb etc., have long vowels. And interesting thing is that spelling remained as it was.
The w of sword, two, who etc., was lost in this period of time.
Middle English Period (The Norman Conquest)
When Normans (French) conquered Britain, they brought their own vocabulary to England, in fact, their language displaced Old English and became the language of the official class in England. One of the most obvious effects of the conquest was the introduction of Anglo-Norman. Even, French names displaced Anglo-Saxon names.
It was Middle English period and many other sound changes and shifts took place.
Final n was dropped when it was part of an inflectional syllable (but remained when part of the root, e.g. seven, or in derivational endings, e.g. written) so OE mētan > ME meete(n).
The final schwa was dropped and resulted in the silent e. Several other vowel shifts took place during this period of time. One of the main changes was Trisyllabic Laxing.
i and y were used interchangeably to represent i: myne ('mine'). v and u were indistinguishable etc etc.
Modern English Period
There are many sound changes that took place in Modern English. There were many complex clusters in Old and Middle English that reduced and became simpler in Modern English. The 't' in words like listen, glisten, fasten etc., was pronounced up until Early Modern English but was later on lost. Some other changes (most of which resulted in silent letters):
- kn in word-initial positions became /n/ as in know, knowledge, knight etc.
- gn became /n/ as in gnome, gnat, gnash etc.
- wr became /r/ as in write, wrong, wrath etc.
- gh in words like knight, might, night went silent, lengthening the preceding vowel.
- silent t in words like glisten, listen, often, whistle, Christmas etc.
- The -ed endings were pronounced with separate syllable in Middle English, they came to be a part of the root word.
The Great Vowel Shift
The Great Vowel Shift was a series of changes in English vowels that took place between 1400 and 1700. Through this vowel shift, the pronunciation of all Middle English long vowels was changed. Bite used to pronounced the same way as you pronounced beet in Modern English, boat had the same vowel as Modern English thought (/ɔː/) etc.
Many of these sound changes and shifts changed pronunciation but the spelling didn't change, which is why Modern English spelling isn't congruous with Modern English pronunciation.
Borrowings from other languages
English has borrowed loads of words from other languages.
The introduction of Printing Press contributed enormously to the idiosyncratic spelling of English.
Read this answer for details about the ch.
I've used ⟨angled brackets⟩ for spelling, /slashes/ and [square brackets] for pronunciation