Light propagates in straight lines because it follows the principle of rectilinear propagation. While light waves do exhibit a spherical wavefront when they are emitted from a point source, the overall behavior of light can be understood by considering the superposition of these spherical wavefronts.
When a wavefront propagates outward from a point source, it expands in a spherical manner. However, the wavefront itself is not the direction of travel of light but rather a representation of the positions of points in the wave that are in phase with each other. The direction of travel of light is given by the ray perpendicular to the wavefront.
Consider a point on a wavefront, and imagine drawing a line tangent to the wavefront at that point. This tangent line represents the direction in which the wave is propagating at that particular point. Since the wavefront is spherical, the tangent line at any point on the wavefront will be perpendicular to the radius of the sphere passing through that point.
As the wavefront propagates outward, the tangent lines at different points on the wavefront form a set of rays that are all perpendicular to their respective wavefronts. These rays collectively form the path along which light travels.
So, while the wavefront itself is spherical, the direction of travel of light is perpendicular to the wavefront, following the principle of rectilinear propagation.