There's already over 30 pages of multiplayer tutorials, covering concepts, a chat room, pong and a real-time game. We can't, and never will be able to, make a dedicated tutorial for every kind of game anyone might ever want to make. Besides, it's better to teach the fundamentals, so you learn how things work and how to build new things yourself, rather than just following recipe-style tutorials.
Honestly, I think there aren't that many people using multiplayer because networking is just fundamentally difficult. You can make all sorts of excuses, but ultimately you are going from one game (which is often hard enough!), to multiple simultaneous games, all with slightly different time offsets, and all of which need to communicate with each other with varying degrees of latency, latency jitter, and bandwidth constraints. It's far more complicated and requires a whole different mental model about how to design it, which can take a long time to get comfortable with.
So yeah, multiplayer is just hard. I warned everyone it would be, but I think hype exceeded reality, and then when everyone realised that actually yes, it is hard, then it ended up with less usage.