Ok sure, but earlier you were stating that 'cinematic' was in some way opposed to 'realistic'. But surely at least some films made according to those 100 most powerful film conventions have an air of gritty realism?
I think you're confusing the use of 'realism' regarding PC power level (as in, PCs are only as powerful as real-life people might be) with 'realism' in the sense of real-life instances of random chance being "stranger than fiction".
There are many real-life stories that, while being unbelievable personal anecdotes or news items, fail to translate to entertainment media. The reason for this is that random chance can ruin an otherwise good story. So while
true stories about random chance saving the day might excite us, the same mechanism of resolution in fiction is immensely dissatisfying - and is known as
deus ex machina.
So the main reason grim'n'gritty play is anti-cinematic is because users of a grim'n'gritty system almost exclusively desire to remove the advantage to PCs inherent in just about every system and replace it with more random elements.
In much the same way, sandbox play removes the traditional story structure from the PCs' adventures and replaces it with the same organisational force that ultimately guides our real-life day-to-day experience - random chance.
In cinematic terms, this doesn't provide for good storylines because good stories, unlike real-life, are organised along specific lines and contain specific elements - beginning, middle, end, set-up, payoff, rising action, turning point, denouement, etc, etc
The more narrative-style adventure design espoused by most published game designers (and so scorned by sandboxers) makes it more likely that these dramatic elements will occur.
And even more to the point, is it really a convention of good 'cinematic' role play that the heroes always enter stage left? Is that what you were talking about when you said, "modelling the conventions of drama more than modelling the conventions of real life"?
See above.