They lack any meaningful design in comparison to other currencies, since all denominations stick to mainly black and white designs with slight hints of color as they’ve remained the same for a long time. Why won’t USD switch to Polymer bills (it was first unveiled in Australia) like they’ve done in the UK or UAE for example.
IIRC—and I may not be, I can’t recall where I ran across this—it’s partly due to the U.S. dollar having been the world’s reserve currency for so many decades. A staid and sober design was seen to play into the image of a ‘serious’ currency.