Because drinking isn't dangerous, and driving isn't dangerous – at least, not in comparison to the single activity "drinking and driving" – the two words are treated as a single unit.  

This might happen a lot when we combine two or more elements in a sentence: 

> My favorite drinks are gin and tonic. 

This could mean I have two favorite drinks: gin is a favorite drink, and so is tonic. 

> My favorite drink is gin and tonic. 

This means that a gin-and-tonic is my favorite drink.