Why do you think it is such a challenge for secularism to find a basis for assigning human beings innate worth and dignity?

“While it has a been a perennial challenge for secularism to find a basis upon which to assign human beings intrinsic worth and dignity, the concept of human dignity and intrinsic value (including its implied ethical principles such as respect for persons, etc.) is inherent biblical teaching and Christian tradition.”
 
Why do you think it is such a challenge for secularism to find a basis for assigning human beings innate worth and dignity?