I'm not sure what you mean by "manufacturers" for alcohol. When I think of "alcohol manufacturers" I think of chemical-grade alcohols for industrial or research use, such as Sigma Aldrich. Looking only at high-purity ethanol, different products are packaged in 1L glass bottles, 7L stainless steel drums, and 20 L polymer (plastic) containers.
My guess is that their decision to sell 1L containers in glass bottles has to do with the economics of obtaining glass vs. high-quality plastics that will be stable in standard laboratory operating conditions (i.e. no container breakdown) and the economics of shipping.
If, on the other hand, you are inquiring about consumer liquor manufacturers, you can still see a variety of packaging methods. Beer (an alcohol product) often comes in aluminum and steel cans. Wines may be purchased either in plastic bags (typically packaged within cardboard boxes, primarily for bulk sales) or in glass bottles. Hard liquors often come in glass bottles, but many brands are switching to using hard plastic containers.
While plastics are capable of replacing much glass, there are some significant factors to consider in the conversion. First, glass remelting is somewhat easier than plastic recycling. Second, the price of glass is not as volatile as that of plastic because the raw materials of plastics come from the oil industry. Third, and probably most important, consumers are used to glass packaging and are demonstratedly wary of using alternative packagings, regardless of proven efficacy and safety.
This is particularly true of wines and liquors with expected "short" shelf lives of only a few years; as all plastics are prone eventually to thermal and ultraviolet degredation, it is unlikely that fine wines or things desired to be stored for long periods of time (such as 18-year malt liquors) will experience packaging changes. The rate of breakdown is dependent on the type of plastic, however, and many plastics operate well for several years with no detectable deterioration, making them safe storage containers for the majority of alcohol uses(despite much public misinformation, as can be seen even among the other answers on this page).