Why is static friction not a single solid number

Just like the coefficient of kinetic friction introduced in the previous module, the coefficient of static friction is a dimensionless constant that characterizes the nature of the contact between two surfaces. ... When static friction is acting, it is generally less than its maximum value.

Explanation:

Static friction, acting in the direction you're walking. Kinetic friction, acting opposite to the way you're walking. Kinetic friction, acting in the direction you're walking. When static friction is internal, however, it is a non-dissipative force, performing zero net work on the chosen system. The reason is that static friction guarantees that the two interacting objects will undergo the same displacement. Thus, the work done on the two objects will cancel by Newton's Third Law. Static friction is the force of friction on an object that is not moving. If you push on a stationary block and it doesn't move, it is being held by static friction which is equal and opposite to your push. If you observe the application of friction from a frame fixed to any one of the surfaces on which friction is acting, work can never be positive. Friction, by its nature, always acts opposing the relative motion between two surfaces. work done by friction can be zero, when there is no relative motion.

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.