Why do academics drink so much coffee?
Academics sure do drink a lot of coffee---but do they drink any more coffee than other professionals?  

According to Wikipedia, 150 million Americans adults drink coffee daily (out of about 240 million total), a rate of more than 60%.  So if most adults drink coffee in America, it's entirely unsurprising that academics are like most other adults and that departments might happily supply this cheap perk---just like many, many other offices do.  Similarly high rates of consumption appear to pertain in most other developed Western countries, which are also where the majority of high-impact scientific research is still conducted.

In short: many academics appear to drink a lot of coffee simply because they are typical adults in societies where most people drink a lot of coffee.