Dictionaries do not tailor their information to a specific group of learners. They need to be general - after all, even native speakers use dictionaries, and it is unfair to be told that your way of saying something is somehow invalid, or less valid. If you have an issue with the way a dictionary provides information, you could complain to that dictionary! There is not even a 'standard' American accent. There can be considerable regional differences in the way people pronounce things around the USA. But, having said that, not all pronunciation differences are regional either. Some words are said in different ways depending on context. e.g. 'the' https://www.quickanddirtytips.com/education/grammar/how-pronounce An explanation like that is outside the scope of what a dictionary should provide. The dictionary probably simply says that there are two pronunciations, and if you are interested, you need to work out for yourself when to use them. If you are only using IPA to learn, you may not be learning very efficiently either. Most people learn best by listening, and copying. To me, IPA is more useful to help you understand what people are saying, rather than as a way to learn in itself.

"American accent is the major accent that all international students have been learning" -- actually no, it's not. Of the 1.2 billion English speakers in the world, only 25% speak the American dialect. Here in Europe -- where there are more English speakers than the US and Canada combined -- everyone learns the UK dialect at school.