Accessibility. Plus the cutscenes are disjointed because they're obviously segmented by gameplay. Everglow is an amazing example of what you can get if you were to streamline all of the events so far in the right order, but that's a huge undertaking and it's still not the easiest method of getting into the series.
Plus, there's a difference in what they can do as an actual TV series vs. only cutscenes. There's a lot of possibilities that could be done with this format. Idunno if you've seen this thread, but there was one a while ago where people wanted a TV series for Kingdom Hearts where it's essentially a slice-of-life anime. So stuff like Sora, Kairi and Riku chilling by the beach, RAX hanging out in Twilight Town with Hayner, Pence and Olette. TAV having random hijinks at the Land of Departure. With a series, you can get away with this. In a game, you can't, because pacing is a issue.
Imagine an episode where Roxas is having sleeping problems, so he goes to Ven because... y'know... and ridiculousness ensues. Or Axel having a crush on Aqua, but he doesn't know what to say to her, so he asks Kairi and Xion for advice (two who have absolutely zero experience in this). They're silly ideas, but that's honestly all it could be! Just little shorts, similar to RWBY Chibi.