[a_random_canuck]

It’s not that it needs to be, it’s just that—based on our knowledge of the chemical elements—life based on some combination of oxygen, carbon, hydrogen, and nitrogen is by far the most likely.

Other forms are certainly possible, it’s just that they are highly highly unlikely to form and remain stable, and develop into complex intelligent life.

So all of that informs where you should focus your efforts on research and searching.