If cigarettes and alcohol are do dangerous, why are they legal?

Excellent question: Probably because they have a longer history of use and are more socially acceptable. But both tobacco and alcohol cause a tremendous amount of disease and are by no means safe.

Use and lobbies: These are traditional products, available long before heath and other risks have reached the public forum. They exist in the market because they are addictive and have a huge following, they are agricultural products with a significant lobby, they are a tremendous source of tax revenue and history shows that attempts to eliminate them have only made the problem worse - e.g. Prohibition.