Simplicity, price, they only require the camera to out last the warranty period.
A workaround to the case of a dead battery and empty volatile memory would be to include a fail-safe mode in the camera firmware to load the desired configuration settings from a file on the memory card instead of simply assuming hard-coded defaults. And the proper time could be obtained from the gps receiver. In the worst case scenario of not having a gps receiver or not yet having acquired a gps signal, the dashcam could read the most recently dated filename of video recording on the memory card and increment by recording file interval time so as not to overwrite existing saved files on the memory card. Though the time would not be correct, at least the dashcam would continue to function.
But as someone already mentioned, Viofo will instead want to sell you a new dashcam when that time comes. So why bother programming now for the inevitable case of a dead battery later...