For the same reasons that make any peace of art valuable!
Rug weaving is an art that is passed through generations and weaving a piece, can take up to several years.
A hand knotted rug is a rare form of art that goes back thousands of years, very few other arts can match that and very few artists in the entire world can put in the amount of effort it requires to produce one.
Nowadays, you can buy machine made rugs in the same way that you can buy a poster of Mona Lisa. Much like the original Mona Lisa, hand made rugs are incredibly more precious than the machine made one.
Just take a look at this magnificent rug:
It's beauty, details, elegance and the skills that makes Persian rugs so valuable.