The very important thing in modern internet media: parasociality with the audience. Only by being live can you have a conversation with "chat", whether that's individual chatters or the hivemind of very large audience streams.

There is a special skill of streamers who can play an action game on one half of the screen and read enough of chat on the other half to make and respond to jokes, at the same time.

Another consideration: youtube and twitch apply different music licensing considerations to live, so almost anything that involves music MUST be live and unarchived.