Why is a ladder an inclined plane?
By definition, as listed on Wikipedia: "An inclined plane is a flat supporting surface tilted at an angle, with one end higher than the other, used as an aid for raising or lowering a load."
This is exactly how we use a ladder. Whether the load is us, or something that we are carrying, we use the ladder to raise or lower the load. Angling the ladder closer to the horizontal increases the length of the ladder needed, but it greatly increases the mechanical advantage.
Here is a very short video that explains inclined planes very well: