Probably for the same reason that doors are on loos and bathrooms. It doesn't seem right, or accepted by society, to bath, go to the loo, burp, or fart, in public.

They prefer Good Manners.

1) because of the unpleasant odor: "The gas released during a flatus event frequently has an unpleasant odor which mainly results from low molecular weight fatty acids such as butyric acid (rancid butter smell) and reduced sulphur compounds such as hydrogen sulphide (rotten egg smell) and carbonyl sulphide that are the result of protein breakdown. The incidence of odoriferous compounds in flatulence emissions increases from herbivores, such as cattle, through omnivores to carnivorous species, such as cats." 2) "In many cultures, human flatulence in public is regarded as embarrassing but, depending on context, can also be considered humorous. People will often strain to hold in the passing of gas when in polite company, or position themselves to conceal the noise and scent. In other cultures it may be no more embarrassing than coughing. While the act of passing flatus is generally considered to be an unfortunate occurrence in public settings, flatulence may, in casual circumstances and especially among children, be used as either a humorous supplement to a joke ("pull my finger"), or as a comic activity in and of itself. The social acceptability of flatulence-based humor in entertainment and the mass media varies over the course of time and between cultures." Source and further information: http://en.wikipedia.org/wiki/Flatulence 3) The odor could also be a reason by burping: "Belching, also known as burping, ructus, or eructation, involves the release of gas from the digestive tract (mainly esophagus and stomach) through the mouth. It is usually accompanied with a typical sound and, at times, an odor." "In many parts of the world, audible burping is considered highly impolite and childish. Public belching tends to be received in a manner similar to flatulence. However, in some cultures, belching after a meal is considered a polite way to express appreciation." Source and further information: http://en.wikipedia.org/wiki/Belching

Some things that are natural and sometimes even very enjoyable are best done behind closed doors both for the sake of manners and privacy.