Why transfer functions are always defined under zero initial conditions ?

We use LTI systems in practice. That means the system should be LINEAR. Zero initial conditions ensure Linearity. That is why, Initial Conditions are assumed to be Zero in transfer function model.

The systems should be linear. Zero Initial Conditions ensure linearity.

T.F IS ONLY APPLICABLE TO LTI SYSTEM WITH ZERO INITIAL CONDITIONS.BECAUSE IN CASE OF NONLINEAR SYSTEM INITIAL CONDITIONS ARE NON ZERO   AND IT IS DIFFICULT TO FIND T.F.