Not everyone has a good enough computer for this.
There's no good way to have an in game way to convert stuff. At best someone could set up a website or smth where a bot takes replay files, inputs them into the game instance, and then spits it back out for you to download as an MP4. But that'd be an exorbitant amount of effort, and wouldn't be official. But I theorize it'd be the most reasonable option regardless.