A child find it hard to draw realistic 3D perspectives, because her brain is still developing 3D perception. She draws a house with a front, and a side wall without that wall disappearing towards the horizon. She is not drawing what she sees, she is drawing what she thinks the house looks like. When the drawing is finished, she notices that someone is not right, but she finds it impossible to lay her hand on it what it exactly is.

The same is true for grown ups and graphics design. You see a beautifully designed page, you want to make something similar in PowerPoint and somehow, it does not come out. Why? Because you stuck to your own mental model of a PowerPoint slide (and what you think it should look like) and did not really see how the designer deployed white space, used of grey scales in text rather than blunt black, and set the space between title lines slightly tighter, and was careful not to overdo it with the colours.

Here is an exercise. Take a poster or design that you really like and literally recreate it in PowerPoint (or Keynote) until it looks exactly the same. Now apply that template to your presentation.