And if the answer is "because they always have", it comes very close to being correct. The business model predates Prohibition, so unless you are reaaaally old, you probably don't remember going to a bar before 1920.
A bar, or a saloon, as they were more commonly called before Prohibition, was a place of trust. The drug, alcohol, was dispensed by the bartender, which whom as much confidence was placed in as an illegal drug dealer is today. If they wanted to stay in business, they charged a fair amount, but didn't gouge their customers. It was a drug, but it was a legal drug.
When Prohibition went into effect in the United States, in 1920, the price of alcohol, like all illegal drugs, soared.
After repeal of prohibition, the price of alcohol dropped dramatically. If you wanted to buy some illegal "hooch" during prohibition, you not only paid a lot, you had to do a fair amount of sneaking around. Once it became legal again in the United States, as is common with all supply and demand, the supply increased dramatically, and the price dropped. So much so that it would be the equivalent of gas dropping down from over three dollars a gallon to just a few pennies. Who would ask the price? It was reasonable!
Eighty years after the end of prohibition, bars are still in that cheerful mentality. Asking the price of a drink at a bar is a serious social mistake! And also, in addition to paying whatever the bar decides to charge (which can vary widely based on if the bar is "trendy" or not), there is the need to add at least twenty percent to the asking price.
The only time that you will see prices posted at bars is during specials, for example $2 beers. Happy hours will advertise half-price drinks, but the full price is never, ever, posted.
This abusive business model has not gotten any attention from government agencies that normally keep the public from being cheated. The general feeling has been, and continues to be, if people choose to drink alcohol in public, the more money that is charged the better. It limits most people's consumption, and it makes a LOT of money for the businesses, and the community.