Why do far away moving objects appear to move slowly

Question

Whether we see a far away car moving at the same speed as a near by car , or we see the road and far away landscape (mountains) moving (backwards) when we move in a car , the far away object always seems to be moving very slowly. I did not ask this on Physics because I think it's answer should be a mathematical proof. It has to be due the bodies rate of change of distance. I hope angle is the suspect.

Answer 1

suppose an object is moving at $x$ meter/second horizontally, and this object is moving $1$ metre height above the observer.

So, after one metre, the object can be seen at an angle $\theta_1$ and
$\theta_1 = \tan^{-1}\left(\dfrac{x}{1}\right)$

Suppose the object was moving $100$ metre height above the observer. then,
$\theta_2 = \tan^{-1}\left(\dfrac{x}{100}\right)$

Here, $\theta_2 \lt \theta_1$ means, the heigher the height of the object is, the lesser its speed is to an oberver