Why is dietary fiber considered not a nutrient?

Fiber is considered a nutrient. It's a dietary component that is essential for good health. Your body doesn't actually breakdown insoluble fiber, so you can't absorb the caloric content. It's still very beneficial.
Protein, carbs, lipids (fats & oils), vitamins, minerals and in some cases

water are considered to be nutrients.

Fibre is also an essential component of our diet.

However, in metabolic sense, fibre cannot be considered a nutrient

because it is not digested nor absorbed by the body.

I hope that answers your quetion.