Why does all timecode start at 1 hour? (01:00:00:00)
It's to leave room before first picture, for slates, countdowns, test signals etc, while still preserving an easy count of running time for the video. Yes, it traces historically to broadcast and in particular videotape.

Many tape-based editing systems couldn't deal with 24-hour wraparound, so the next even hour became traditional 'time zero'. I suspect that most non-linear systems are the same.