Question

why does light travel in a staright path?

Answer 1

light travels in straight line in vaccum and because it follows the property of rectilinear propagation

Answer 2

This is a good and intellectual question you have asked.
there are many misconceptions related to this topic, but I will give you a brief idea about this.

Light is an electromagnetic wave or radiation which does not pose any requirements of a medium to propagate.
light is a transverse wave, which means that it oscillates perpendicular to the direction of propagation.

Light doesn’t travel in straight line always. It’s path depend on refractive index. Inside constant refractive index material, light travels in straight line.

 It is a traditional view that light travels in straight line.

I can give two examples which shows that light can bend:

1) Diffraction
2) Gravitational lensing

Light travels in the straight line because it does not generally interact with the medium and wherever it interacts it does change its path like when it is entering a different medium with different permeability and permittivity (denser or rarer). This interaction and bending of light is called refraction.

I hope it helps you ^_^