There may be a hundred of answers to this question, but for me, perhaps, one reason is the fact that when we learn to speak English, we learn how to speak it at the same time, too. When we learn a specific word and its pronunciation, we learn it according to how it sounds to our ears, we try to repeat exactly how we hear it, and most importantly, we accept how it sounds, no matter how strange it is to our ears, wholeheartedly. We do not concern ourselves with the spelling, IPA symbols, position of the tongue, the shape of the lips and how other speech organs work to produce the sound.
Another factor may probably be the fact that we are exposed to English sounds at an early age. We hear them anywhere like on the radio, from our older siblings, from our neighbors and when we watch TV, we hear them, too.
And another reason why it seems relatively easy for us to learn English Pronunciation and the language itself is that we are quite flexible and quick to adapt. This is not only in the case of English language, but other languages as well. One sound proof of it is the presence of Filipino people in almost all nations.
Well, I hope this is an insightful answer. Thank you for reading.