Question

give reason why the international date line doesnt folllow 180 degree latitude exactly

Answer 1

Since the International Date line is completely political. It was set for the convenience for the nation states it passes around, hence the crookedness. It roughly follows the 180 degree longitude line. That line is opposite the politically set 0 degree longitude or prime meridian in Greenwich, UK.

Since it would be inconvenient to have countries split between different days, the line tries to miss them and make life a bit easier.