I assume in order to realize this greatest technological achievement, people working on it must have been trying their best. Trying your best includes not wasting time and resources on producing an extensive(and completely useless) video log of every step taken. Especially the construction in space, I imagine, would've only involved cameras that were absolutely necessary for the process, with storage and communication capabilities optimized for specific purpose, so any footage (if) preserved would've been incidental not documentary.
I think this is obvious and not surprising. It's not like there were people living next door in the orbit, that could have taken some videos on their camcorders. If there were, I'm pretty sure they would have been recruited to drop everything they were doing and help build the ISS.
