Because Design is most markedly not science. I think you’ll find that in the engineering design fields (such as product design; electrical design; mechanical design etc…) a scientific approach is often used, though perhaps not properly or stringently. In other fields of design (industrial, fashion, interior) the curriculum taught and the business practices don’t support or encourage scientific methods. It’s empirical at best, and non existant at worst.
Most art-based design students and practitioners flee conventional scientific boundaries, as they are aesthetic and “feel” driven. This is a real challenge for most companies, as usability and product testing often suffers. Take the case of the cell phone manufacturer Samsung. They have been turning out some fine new designs, and using interesting materials. However, their quality control and testing is abysmal - the same can be said of Nokia, and many others.
Consider the following statement: “Applying science to design takes the fun out of it”.
The real issue lay with combining these two disciplines - art and science - into workable products. Most companies fail at this goal due to reasons far beyond the scope of this thread. It’s partly in the dynamic of getting engineers and artists to work together.