Why is it so difficult for many toxins and poisons to penetrate the blood brain barrier?

The blood-brain barrier (BBB) is a highly selective permeability barrier formed by tightly packed endothelial cells in the brain's capillaries, which restricts the passage of most substances, including many toxins and poisons. This barrier is crucial for protecting the brain from harmful agents while allowing essential nutrients to pass through. Additionally, specialized transport mechanisms exist to facilitate the entry of vital molecules, further complicating the ability of many harmful substances to penetrate the BBB. Consequently, only certain small, lipophilic, or actively transported compounds can cross this barrier effectively.