a movie is different from talking to a group of people because it's a completely different genre. A movie is a story while a speech is oratory.

a movie IS all storytelling, and for a movie (or novel) to hinge on a single speech is bad story telling. people aren't fickle enough to immediately change their ways immediately after hearing someone speak, without time for reflection and reaction. therefore, it's not something readers generally accept in a work.
what if Dumbledoor talked sense into voldemort in a speech and they all lived happily ever after? you'd be dissatisfied right? because it's completely ludicrous to have people change their minds like that.

there's a huge difference between a shakespearian play and a modern movie. first of all, a play is dialogic, not oratory. the interaction between characters is what drives action, along with internal reflection during monologues (this is what's missing in movies after speeches, instead of the traditional cheering scene, but that wouldn't work either). in an old play, your suspension of disbelief depends on visually imaging what's there, women being played by men, a minimal amount of props and costumes, such that a character generally says something to the effect of "who sneaks there?" upon which the ghost says "i am the ghost of blablabla" because that's how the audience would find out. A modern movie does not have any of those limitations, but can emulate reality very closely in terms of the environment.

in real life, do you see yourself switching from being a firm democrat to becoming a republican instantaneously after hearing Romney speak a single speech? it's absurd right? that's not how a speech works. so why in the world should it work that way in a movie? a "speech scene" is just a very easy way of spotting a dramaturgic point where there HAS to be a turning point, but there is no real mechanism of character development to facilitate that change in future action. the movie is bad because the director hasn't figured out how it all connects together, so they've used the cheap "speech and unrealistic effect" to salvage the wreckage.

Now what if you had the sports coach lay out a strategy that would let his underdog team beat their opponents after being miles behind? you'd break down into sports technicalities and would lose the audience. if you don't lose the audience, the story still turns out to be a story that's not worth telling, because the reaction of a speech that suggests moral superiority of the underdog team gives the audience the personal investment to make them FEEL the team deserves winning is missing.

the "speech scene" shows the movie is lacking dramaturgically and is incohesive. you keep linking speeches with unbelievable characters that change the cornerstones of their lives after a few minutes and never look back, even though their personalities are cautious, they are principled or behaviorally conservative. that is not how personal change takes place.