Now can you explain to me how the same idea apply to semen? Because it was also considered "unclean", in the same manner menstrual blood was.
For semen, there is a clear distinction that it is anything it touches becomes dirty and needs washing. But for menstruation its whatever the woman touches that becomes unclean (and for seven days it seems!), not merely what the blood touches.
The original comment thus provides an explanation for why these two cases are treated separately, which is basically what you asked for.