She is not saying why. All she is saying is that she is not opening the beer. There is no implication that it is hard. That would be if she said that she couldn't ("אני לא יכולה לפתוח את הבירה")
Maybe she is shocked by the suggestion that she would open the beer. Maybe she's stopped by the police and she's telling the cop she isn't opening the beer (because driving with an open container is a violation). Maybe the emphasis is on "אני" because she thinks somebody else should open the beer.