Why Is It Difficult to Perfectly Eliminate Basis Risk?

It is difficult to perfectly eliminate basis risk because the spot price and the futures price are influenced by different, non-identical factors that are constantly changing. These factors include: unforeseen supply/demand shocks, non-uniform information flow, the cost of carry being non-constant, and the reference rate not perfectly matching the hedger's specific spot price.

The correlation is never perfectly 1.0 over time.