Why do solutions to problems feel simple (or obvious) once we are given them? I was watching a video about one of the hardest maths problems even examined in the IMO (International Math Olympiad). And once I was walked through the solution, it seemed so obvious to me, as I'm sure it did for most people. It had a two step solution and was explained very intuitively.

Another example of this is whenever I watch a movie and replay the story line in my head afterwards, it always feels that the story line is predictable. Almost as if I could write it myself. When in essence it's not doubt difficult and I couldn't never predict it's plot line without seeing the entire film.

Given any outcome to an event (such the answer to a mathematical problem or the plot of a movie (or anything really), why do we feel as if it was so obvious?

Besides the throw-away answer of 'cockiness or 'arrogance', is there a physiological phenomenon for this sense of confidence/ know-how one may experience after being told the solution/end to something.
The closest one that comes to mind is called the Hindsight Bias. Basically this means that things seem to be "more obvious" than it actually is--usually after the event had just happened. 

Definition:

Hindsight bias is a term used in psychology to explain the tendency of people to overestimate their ability to have predicted an outcome that could not possibly have been predicted. In essence, the hindsight bias is sort of like saying "I knew it!" when an outcome (either expected or unexpected) occurs - and the belief that one actually predicted it correctly.

Usually coming up with the idea is difficult, for instance solving a math problem. The challenge is finding the right mental model to approach the problem, but once the solution or approach is presented to you, it may be easy to accomplish and you'd feel it was obvious.

Or it could be really just that predictable or that you're natural/good at the subject (e.g. Math) to begin with and it is easy for you.