Why is the direction of pressure always perpendicular to surface area for fluids?

This is actually something not easily answered because it is part of the definition of pressure. I will instead point you to other answers which hopefully make sense.

The following is an excerpt from Lumen Physics.

*The force exerted on the end of the tank is perpendicular to its inside surface. This direction is because the force is exerted by a static or stationary fluid. We have already seen that fluids cannot withstand shearing (sideways) forces; they cannot exert shearing forces, either. Fluid pressure has no direction, being a scalar quantity. The forces due to pressure have well-defined directions: they are always exerted perpendicular to any surface. (See the tire in Figure , for example.)

Finally, note that pressure is exerted on all surfaces. Swimmers, as well as the tire, feel pressure on all sides. (See Figure 3.)

*

Basically, a key part of the above is saying that pressure is always perpendicular it is because if there were a component of force parallel to the surface, then the object will also exert force on the fluid parallel to it as a consequence of Newton's third law.

Additionally, there is a similar question to physics stackexchange.