Because her party would have to do that, and nobody else is daft enough to want the job. So far.
Having another general election would be disastrous at this point in the Brexit process, so the only way to remove her is for the Conservatives to have a leadership election. To kick this off, either the incumbent leader resigns, or 15% of all Conservative MPs (currently 48) would have to write to the Chairman of the 1922 Committee requesting a vote of no confidence in the leader. If that vote is held and passed, then there is a leadership election, in which the current leader cannot stand.
Now just look at the party. Who else could do the job of trying to hold the party together, get Brexit done and could be guaranteed to do it better? Honestly? You may want to complain about her but what's the alternative? Boris? Gove? Davis? I just can't see anyone who impresses me more than Theresa and in their heart of hearts, so does the party realise that the last thing it wants right now is another big convulsion. She deserves some support and less of the infighting.