system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If we know that the interior angles of all triangles add up to 180 regardless of triangle type, why is it a theorem and not a postulate?

To me, it just feels kind of pointless. Why does it need to be proven that the interior angles add up to 180 when we know that, yes, they're going to add up to 180 in the end?
gpt
How do you know that? Do you understand that it's not true in non-Euclidean geometry? 

At the heart of mathematics is the idea of proving as much as possible using as few postulates as possible. So the main reason for not treating it as a postulate is that we can prove it from simpler postulates. 

An example of the significance of this approach is the Axiom of Choice. It took a fair bit of discussion for mathematicians, way back when, to accept that this needed to be an axiom. The bias in math is for fewer postulates, not more.