I had this question while I was reading the differences between pressure and stress.
As I have read:
Pressure is the intensity of external forces acting on a point, and it always act normal to the surface.
Stress is the intensity of internal resistance force developed at a point, and it could be either normal or parallel.
Ok then, why isn't there a parallel or a shear pressure?
When I thought about an answer, I told myself, by referring to its definition, that it may be because it is acting on a point, so if there is a parallel pressure then it won't be acting on a point but instead it will be acting on a surface. Great then let us do another definition for a parallel pressure.
But then shear stress is acting on a surface and not on a point, so how come that stress is defined to be acting on a point while shear stress opposes this idea?