Imagine you didn't understand the difference between a hardware accelerated CSS3 animation, a JQuery tween or a declarative SVG transition. How would you be able to decide on how your UI moved gracefully from one state to another on a dozen different devices?
Or imagine you couldn't mentally measure an interface feature in kilobytes. How would be able to decide on the fastest experience for a person on a slow connection?
Imagine if you never read the data model documentation and had to guess the presence, function, relationship and structure of each object. How would you decide the composition of an interface?
Imagine if you had to create tactile interface without ever touching it. How would you comprehend what gestures didn't confuse?
Imagine you didn't spend Sunday evening reading through the W3 specifications and browser vendors nightly READMEs. How would you understand what's going to happen next?
Design is a manifestation of experience, and the experience is ever more complex.