One of the main explanations is market forces. Beer advertisers are under increasing pressure to raise their game as the market dries up before their eyes. In the US, a market worth $57bn (£38.9bn), beer sales grew just 1% to 198m barrels in 2000, according to the Beer Institute. In the UK, beer sales are at a 30-year low. Consumption has fallen steadily year on year, dropping by 3.7% in 2000 alone.
In the US, companies face increased competition from imports, which have gained ground over the past five years. According to Mintel research, 31% of consumers prefer imported beer, with Mexico's Corona being the market leader.
In the UK, the shift has been to wine. This a result of the greater variety of wines now available, its increased value for money and change in tastes and habits with more people eating out and drinking wine with their meal.
As well as a declining market, beer clients are also faced with huge competition. There are hundreds of brands vying for consumers' attention, so how do agencies achieve stand-out?
Dennis Ryan, executive creative director of JWT Chicago, has had the enviable task of spending his entire career working on beer. He believes the first step is to identify your positioning in the market.
"Beer is a badge. What beer you drink shows what kind of person you are, so you have to develop advertising specific to that image. Do you have a specific audience or the same audience in the middle that everyone else has?" he says. "Comedy is a consistent favourite of beer advertisers. But can you compete in that?"
David Apicella, executive creative director of Ogilvy New York which has the Miller Lite account, agrees. "Everyone is on the same playing field so you need a strategic point of difference. Or you just have to be funnier."
Comedy is certainly the most common theme in beer advertising. Typically, it targets males and reflects the sociability and humour of the drinking environment.
In the past, this resulted in the "beer and babes" advertising cliche, where women, if present at all, merely existed to get the guys' attention. This even took place as late as the Nineties, an example being an Old Milwaukee beer advertisement which saw the Swedish Bikini Team parachuting onto a beach. The ad, which was criticised for demeaning women, was withdrawn amid former female employees launching a lawsuit against the company.
Fortunately, those days seem long gone and a new generation of beer commercials is now putting women on top, not only to be politically correct but to target them as consumers. Beer, particularly light beer, is becoming increasingly popular among women. Amstel Light, for example, has one of the highest percentages of female drinkers -- around 45%. A recent ad showed a woman opening a bottle with her teeth and spitting it across the bar.
Bud Light is enjoying huge success with double-digit growth in the US, according to Bob Lachky, Anheuser-Busch's vice-president of brand management. This brand, too, is learning how to appeal to women.
"Women are now on an equal footing and you must reflect that in your advertising. We've learnt the hard way and made some mistakes with sexual stereotypes," he says. "You have to maintain the sexuality but make sure that the woman is not the butt of the joke."
Lachky admits that while the world-famous "whassup?" ads for Budweiser are a "guy thing", they also appeal to women.
"Bud is 85% male but you have to make sure that maleness isn't overpowering. Research showed, however, that women identified with the ads, claiming 'that's my boyfriend'."
For Guinness, it is a question of producing deeper advertising.
"Guinness has moved on significantly from a functional-based property. Now, it's about consumer identity, about self-belief and confidence. But it's still the same values based around the elemental power of the brand," Jon Potter, Guinness global brand director, says.
Where beer advertisers go from here is an issue upon which all our interviewees agree. How do you top up the creative glass when it's already full to overflowing?