Why if I add a pole at high frequencies to an improper transfer function it becomes realizable?
The long answer is here.

The short answer is that the order of the numerator has to be less than or equal to the order of the denominator.

If this weren't the case a small amount of white noise at the input would generate infinite energy at infinite frequency.

Consider the simplest case of a differentiator H(s)=s.  The magnitude response increases without limit, so a physical system would need infinite energy to realize the response.

A real system like an op-amp differentiator for example will of course have poles at higher frequencies.