Why does every math class throughout high school and even university ask you to "prove" things without ever showing you how to structure a proof, how you can tell when you have proven a statement, or even where to begin?

I went through university Calculus 1, 2, 3, and Linear Algebra without knowing how to proof (and high school never asked you to prove anything unless you tried to do all the questions in the textbook).

Literally the first time that they finally teach you boolean logic and the rules of inference, and then build it up to proofs is discrete math, which is almost solely take by CS majors. That means in the other 20 or so different math courses, of which any given student at a university takes at least one or more, students may be asked to write proofs without knowing how.

Can someone please explain the reasoning behind this?