Because a non-native speaker of any language depends much more upon a clear pronunciation of the individual sounds* and on the syllable stress** than native speakers. That is because native speakers’ brains are so used to listening to that particular language that they are able to “fill in the blanks” and understand practically everything they hear, as long as the intonation*** and rhythm**** are right.
It’s quite easy to see this feature in action when you are talking to someone in your own mother tongue. Let’s say you’re at a bar, having a beer with your pals; that’s a pretty noisy environment, but you’re still able to maintain a conversation with them, even if you can’t make out every single word they say, right? That’s because your brain is being kind enough to process the clues of intonation and rhythm, and putting together fragments of words so that you can make some sense out of what you hear. You don’t need your friends to articulate every single sound perfectly, even because after a few beers, that isn’t even possible…
Now, you certainly don’t have that much time to be immersed in an English-speaking environment to allow you brain to achieve that same level of proficiency in puzzle solving. So when you are talking to another English speaker, whether native or not, you pay close attention to the articulation of each sound, and to where the stressed syllable is, and if you hear a sound that doesn’t match the version of a word you have heard and learnt before, you feel like you don’t know that particular word, or you might even get completely lost.
Well, let’s say you’re speaking to another non-native speaker of English, maybe face to face, or over the radio, why not? That person is going through the same process you went through, and may not even have the same linguistic level that you have, so what will come out of that mix? Slower communication, for sure, and possibly a miscommunication. We don’t want that to happen, do we?
So you have to be the agent of change here, and make and effort to pronounce words correctly, to articulate each sound clearly and to imitate the rhythm and intonation of the listening materials available on the market. Because if everyone does their job, we can mitigate the risks of communication breakdowns, which are potentially dangerous in any environment, let alone aviation.
Oh, and before I forget, you can keep your Brazilian accent, all right? Some people are more “musical” and are able to mimic accents easily and naturally and we often feel jealous of that ability. But what we’re trying to achieve is intelligible and clear pronunciation, not a perfect American or British accent. Unless you are born again in the US or in the UK, your mother tongue will always be Portuguese and you shouldn’t be ashamed of that!
Here are some links that will help you improve your pronunciation:
** some call it word stress, it’s the position of the “strong” syllable in a word
*** the rising and falling sounds in a sentence, e.g. rising intonation at thye end of yes/no questions
**** also called sentence stress, it refers to the “strong” words in a sentence, that usually carry its meaning