Because he's a lazy goober who never gets laid. If a guy you have zero interest in talking to offers to buy you a drink, politely decline, then make up some girl excuse to walk away. Go to the bathroom, then come back and find a different spot or just stand in the middle of your lady pack. If he buys you a drink before you can decline, then the polite thing to do is have a conversation with him. After all, a bar is a public space. Your being there does suggest that you're open to talking to other people. If you wanted privacy, you would have stayed at home. Once the drink is finished, thank him and find someone else to talk to, or if you don't want to feel like you're in his debt, buy him a drink, continue the conversation, and then say "It's been fun, but I have to go." If he asks for your number, give him your e-mail address. Going to a bar or a club requires that you practice nice ways to turn people down.
And it helps to see things from the dude point of view. There are guys out there who might ask why every single woman expects them to buy a drink. Keep in mind, that bars and clubs are environments where men are expected to do the approaching, and women have the power to reject. There are guys out there who resent this set-up, and become aggressive about it. They become impatient, and assume that a drink buys them something more than small talk. A drink buys nothing more than courtesy.
In the future, don't accept the offer of a drink. Instead, strike up a conversation and size the guy up. If he smiles, listens to you, and uses words with multiple syllables, then buy him a drink. Or, even better, ask for his number, go out on a date, and let him buy the first round.