I think you are seriously underestimating how important this is, especially when keeping in mind how easy it is. It's not just a coincidence, that so many Youtuber, Companies or any Business really, relies on Social Media. It's the best way to Advertise for free, to connect with your audience, to keep them updated and to give them extra content. And you achieve all of this with relatively low effort.
Not everyone likes using Social Media, in fact I know several Youtuber who were against using platforms like Twitter, as they didn't like it and thought "it's not that important and won't make much of a difference", but all of them started using Twitter at some point and keep it very active as well. It's because it doesn't make sense not to use it in every way possible. It just has advantages. A place like Twitter is also one of the best ways, to get a new audience and new viewers. Someone just has to like or retweet a Tweet, and this will show up in someone else's Timeline. If this person interacts as well, it will appear on their friend's Timeline and this (potentially) starts a chain reaction.
Another thing that people need to keep in mind, the majority of Youtube Viewers do not have accounts. Most of the time, the majority of people who have watched a video, are people who are not subscribed to you. So just relying on Youtube/Twitch Notification is not enough. Social Media accounts are always more common, and thus a great way to remind those people that something is happening. I can see your viewpoint, and I'm personally also not that into Twitter, but trust me, we are the minority.
So yes, it is indeed quite important and the difference that it makes, can (!) make quite a difference. And the more you use it, the more rewarding it will be.