Because its short.

Because the former is a proper mathematical expression, while the latter 
is python jargon with limited use elsewhere.

Exactly, why is there no correct implementation of Julian date in python 
time or datetime?
For most practical purposes I can understand why most people would 
consider the above formula most useful and most elegant.

Then what's the math module good for?

So what? Is this the law of Moses? Or should we fall back to Assebmly?
Python is supposed to be a high-level language not a stone age tool.

However, I am glad you found at least another reason convincing to have 
this function.

Cheers!