Why is it that Jake was able to bear Caliborn? Couldn't he just travel back and prevent Jake's birth?

Jake was able to beat Caliborn because he had big glowy shit, and also because there was a prophecy that he would do so. Caliborn couldn’t just travel back in time because, well, ultimately because that wasn’t the way the alpha timeline went. The alpha goes both ways, Caliborn can no more go against it than anyone else.

I should, however, note one key fact. Caliborn getting beaten up by Jake wasn’t just cathartic and important to Jake, it also led to exactly what Caliborn wanted, since it was only after he was beaten that Dirk sealed him (along with Arquius and half of Gamzee) in Lil Cal and Roxy sealed Lil Cal in the Void, which was the process by which the actual Lord English was created. So, really, getting the snot beaten out of him by an angry kid in tiny pants was the best thing that ever happened to Caliborn, from a certain point of view.