Why??

I am not sure , but I would like to express my opinion on this:

which is purely my understanding and may not be correct--

Why Depreciable assets are always short term, what is the Reason behind that?

Reason: If it is not a short term asset, it will be a Long Term Capital Asset

and for LT Capital Asset, benefit of Indexation is available.

A depreciable asset in most cases,  does not realise an amount equal to its Actual Cost on its Disposal/ Sale.

So it will result in a huge loss after the benefit of Indexation, which when netted of results in reducing the profit earned and thereby a decrease in Tax amount.

Whereas the depreciation has already been allowed as an expense, thereby resulting in double deduction