Like the kids say these days 'Ain't nobody got time for that'. I'd rather watch a 10 min video on Youtube that covers the same topic.
BBC horizon documentaries pretty much took the prize when it came to endless stock video with ambient music and aimless "poetic" quotes from various eccentric scientists talking about some interesting but essentially fantastical topic far removed from any reality or future.
My guess is that the primary challenges are
a) finding a subject that happens to exactly fit the 60-minute time slot without requiring grievous cuts or padding. I imagine it is hard to know this in advance. I notice, for instance, the bigger, famous theatrical documentaries don't exactly have a uniform run-length.
b) production costs. Sure, you could fly around the world to interview another 4 experts and get another 3-hours of footage (that you cut down to 15-minutes) and have a graphics artist who charges $1,000/day generate more models ... or you could just recycle and pad with cheap filler to reach your run-time. After all, you were only given a budget of $X,000 to make a 60-minute documentary.
An interesting example is when Veritasium, who normally does the usual YouTube science content, got some big budget funding for one video [1] and produced it in a style more like what you’d see on TV. It was striking how there was so much filler and dramatization to try to make it interesting, and the whole thing felt dumbed down compared to his usual content, for example [2], which is deeper and relatively information-dense.
And there are channels like PBS Spacetime which go far, far beyond anything I’ve ever seen in any media aimed at a general audience [3].
[1] https://youtu.be/YMDJA4UvXLA