Question

Why human red blood cells do not have nuclei

Answer 1

the absence of nucleus is an adoption of the rbc for itself.
it allows the rbc to carry more haemoglobin which helps in better transportation of oxygen molecules.
it also allows the cell to hold it's bi-concave shape which helps in better diffusion.

Answer 2

with the absence of human rbc the cells can hold their shape which helps in diffusion and they are absent for better movement of haemoglobin