Because of heteronyms.
In most language, you may have words that written differently but pronounced the same (like "ph" is made to sound like "f"), but almost only in English do you have heteronyms, words that are written EXACTLY the same, and pronounced differently.
Like "lead" (what a leader does) and "lead", the soft metal. Or to "live" ("Live long and proper") and a show that is "live".
How to pronounce those words cannot be derived from the spelling, you need the context.
And for many other words, even if they have their own unique spelling, the same group of letters may be pronounced in a massively different way, meaning that you cannot learn how to pronounce from book, unless the word is expanded in phonetic alphabet.