It seems to me that the issue is quite complex. We have become mostly dependent on our senses, because this is how we have been taught to appraise life, especially in the West. So when we look out into the world with our eyes we see people who look different than ourselves and then there is a separation, a sense of "the other." So, yes to your assessment that race is "flimsy and fabricated at a biological level." We wouldn't know that race exists except for how we see things and what we are taught to believe.