This isn't really what "start" is usually intended to mean. They're just identifying the time of the Big Bang, beyond which any possible causal connection is lost and we can't possibly look back further. More time, more space, more something else may have existed and been causally prior, but we can't meaningfully talk about it except speculatively. It isn't part of our spacetime.
Of course, people do speculate. I seem to recall some level of anisotropy in the cosmic microwave background that was a bit more than expected purely from quantum vacuum fluctuations in the pre-inflationary early universe and at least one physicist musing that it might be a perturbation from some other universe that has since lost causal connection. This, of course, makes no testable predictions, can't be falsified, isn't really science, but human intellectual curiosity goes beyond science.
Of course, people do speculate. I seem to recall some level of anisotropy in the cosmic microwave background that was a bit more than expected purely from quantum vacuum fluctuations in the pre-inflationary early universe and at least one physicist musing that it might be a perturbation from some other universe that has since lost causal connection. This, of course, makes no testable predictions, can't be falsified, isn't really science, but human intellectual curiosity goes beyond science.