Because we, the electorate, put her there. Not directly perhaps, but anyone who is prime minister is 1) a directly elected MP, 2) appointed as PM by other directly elected MPs responding to the political situation created by how the electorate has voted. It rarely reflects the opinions of everybody, but tough luck that's democracy.
Look at what happened in 2010 when liberal voters started whinging about them forming a coalition with the Tories. Well duh whatever did they expect to happen given the results?