English as a language includes some one million words, and many of them are pronounced in a way strikingly different from the spelling of the word. There are historical reasons for this, of course: when the English spelling system became standardised it reflected the pronunciation of a certain geographical region in England. English spellings have changed slowly, whilst phonological changes have been much faster. For example, the Great Vowel Shift has occurred, and many newly borrowed words (and older loanwords, as well), retain a pronunciation that is alien to English (e.g. ballet, garage).
The fact that the pronunciation of English words tends to be so irregular gives rise to serious problems for many learners of English. Even very common words (e.g. colonel, bury, yacht) used in everyday speech by native speakers are often irregular when it comes to the orthoepy: you simply cannot guess the pronunciation from the spelling, you must have heard the word pronounced to be able to pronounce it properly. The question is: where do non-native speakers of English get to hear enough English to pick up on proper pronunciations? In many countries much more emphasis is laid on writing than on pronunciation within the domain of EFL-teaching which can be quite problematic.
In addition to everyday words people need to pronounce proper and geographical names (e.g. River Thames, Macleod) and less common words (e.g. baba ganoush, bijouterie) in order to communicate efficiently.
I leave it up to you to think of how learners of English could achieve a near-perfect pronunciation of the core vocabulary of English.