Why? Because a frame of reference moving at a constant velocity is indistinguishable from one at rest. If there were no windows in the train, you wouldn’t even know you were moving.
But if you are standing on a playground carousel and you try to throw a ball to your friend, it doesn’t work right. The ball seems to curve away from its target, opposite the direction of motion. Why?
The “correct” answer is that the carousel is not a uniformly moving frame of reference. But what is so special about angular motion that makes it non-uniform and makes the ball curve away?
It seems like Newton’s laws of inertia are mere descriptions without explanatory power if they just arbitrarily declare that angular motion “doesn’t count” as inertial motion. That doesn’t explain why the ball on the carousel curves away.