First, television and movie production is not "agile". It's waterfall. The script is the design.
Live theater is "agile"; theatrical directors go into a rehearsal room with a group of actors to have a debug session. What comes out may be very different from what went in. In that environment, "I know it when I see it" can sometimes be found by trying different things. You do that in TV or movies and the cost goes through the roof as the production people cope with the changes. Theatrical directors who move to Hollywood sometimes have real trouble with this.
This used to be less true. As projects became more set and effects heavy, more and more is locked in before the first shot. Hence the "previz" business of making the project twice, once in a crude storyboard form and again for real.
Second, there's no patching the product after release. There's no "pivoting". You only get one chance to make a first impression. Although sometimes, movies and TV shows are tested on test audiences and then re-cut.