Because it's not that simple and Biden is loathe to take a step he feels is that drastic.
He would need his picks to pass the Senate, which is not firmly in Democratic hands, and then the rebalanced court would take years to take cases to reconsider faulty earlier rulings.
So unfortunately I think Biden is correct - this needs to be solved with legislation, and yeah it should've been a long time ago, but abortion is such a hot button issue nobody has taken the nerve to do so, and now we are paying the price.
We need a functioning Legislature and currently don't have it.