Because they don't have the evidence or it's unobtainable (either too sensitive or doesn't exist). If China really has a chip that can reprogram/modify server

Because they don't have the evidence or it's unobtainable (either too sensitive or doesn't exist). If China really has a chip that can reprogram/modify server motherboard instructions with only 3 pins all within the size of a rice grain (according to Bloomberg's story), they've already won the tech war.