Well yeah, but when they do design them they look at other websites that are similar to what they want, and that has an effect on the design. I still see some pretty creative websites out there thou...
From a designers stand point, when one looks at a great design, the best part, a concept is taken and stored in the mind. When many of these elements are collected, a good designer can combine them and create something unique. Also, one good element will spark an idea in a creative mind that will inspire to create unique works. So inspiration is a random idea generator in a way. That is why it is recommended that designers collect ideas and look at inspiring pieces of great design before starting work on a project.