Why are people from US and Canada are called native English speakers?

Native simply is a term for natural speaker of the language say for instance speaking English since birth, thus more likely to be more familiar with the colloquialisms and so forth.

It just means that they've been speaking it since birth and so understand it better.

They would know the slang and jargon that is used widely in their countries.

It has to do with speaking it from the infancy. It doesn´t mean that they speak well

Native simply means that the language is something they were taught when learning to speak, read, and wrtie. It doesn't necessarily mean the person is proficient in the intricacies of the language, but it does mean that the person would understand short hand abbreviations, slang and other such jargon better. English, like several other languages, is part of the Germanic language group. English is spoken differently depending on the country. People in the UK, for instance, use different terminology for things. It's just a matter of how our ancestors migrated around. America, for instance, uses English words that have heavy influences from different languages, based on those who came to settle here. The Irish, Polish, German, Italian...

@biooss I suppose a native speaker is one who has spoken it since birth rather than one who is native to the country where the language originated. But yes, the word native is a bit deceptive here. Curious that I am Irish, live in Ireland and speak Irish but am not a native speaker since I did not speak it as a child.

Native English speakers would have grown up talking English in their family. This would be in the UK, Ireland, the USA, Canada, Australia, New Zealand or South Africa. Some people talk British English and some other people speak American English. English is spoken in Hong Kong in schools I think.

It is meant to denote that English is their first language,and maybe it is the predominate language of the country they were born in.

The countries that you mentioned here are all countries where English is the first national language, and the majority of the population speaks it very well.