You vacuum, you vacuum again, over and over you vacuum. The hair is simply stuck in the weave of the rug. Here's the trick -- spray the rug or other fabric with water and then vacuum. It is a tremendous improvement. See the pictures below of a rug we have in the garage; as an example.
Above is after vacuuming three times.
After spraying with water.
See the difference from one side to the other?
Just wanted to share.