Having a "North America" implies there's an "America", it doesn't come out of the blue. You didn't start with a North America, you start with America and then divide it between north and south.

The singular America that you’re referring to that in fact did not come out of the blue was a man by the name of Amerigo Vespucci. He is credited with discovering what we now know as the two distinct continents of North America and South America, together referred to as “The Americas” (Plural) which were named after him.

If there's no such thing as an "America", then why have a northern america?

Because Amerigo “discovered” two continents. One is further north, the other south. Hence, North America, and South America. You’re also confusing terms. There’s a difference between “North America” and “Northern America”. Similar to how there is a big difference between “North Ireland” and “Northern Ireland”. These are not interchangeable terms.

Why is everyone reacting like I'm claiming Canada is part of the US?

They’re reacting like you’re claiming that the astronaut is “American”, which you are, and he isn’t. He’s North American, or Canadian. If you just say “American” then you’re referring to somebody from the United States, which he isn’t. You’re acting like the word “North” in “North American” can be dropped in this context without changing the meaning but you’re incorrect.

Because how can you not be a non-American if America doesn't exist? Good day Sir!

Because “American” is the recognized demonym for the citizens of the country United States of America. It refers to the citizens of that country, not the continent it’s in.

That fact you think you can tell someone else how their culture should work makes you the biggest piece of shit.

My culture and how I experience me and our people own our culture isn't a debate. It's reality.

You don't get to define how my country's people identify ourselves.

Politely, go fuck yourself.

The culture of the people who live in a place takes precedence over the culture of people across an ocean lol.