We don't. But we also don't know that they were, and we don't have enough quality evidence in favor of that hypothesis to justify the confidence with which it is asserted.
In the absence of evidence about the timing being affected by other festivals and in the presence of much evidence that all the actual traditions are far more recent than pagan, I don't believe it's fair to claim Christmas has pagan origins. The absolute best we can do is say that its timing may have been influenced by other, pre-Christian celebrations.
