Minecraft servers these days are literal worlds unto themselves. There are plenty of realms for you to explore if you're a bit adventurous and millions of people to get to know and play with. There are servers that are completely dedicated to building the most stunning works of art you've ever seen, there are multiple different modpacks allowing you to build anything from fully functioning computers to the most glorious cathedrals in Minecraft. And for all these things, there are still countless things that haven't been thought of yet and build. This is what makes Minecraft so special to me. There's quite literally no end to ways of playing the game with new ways of playing cropping up every day.
When Minecraft got released and there was no end, to finish the game, no wither to test your mettle against, there were no beacons to build together with your friends, no one could have predicted that were going to be this many different ways of playing Minecraft thought of. There's literally something for everyone, there are servers that are completely devoted to building the most impressive buildings, plenty of servers would rather focus on the pvp aspects of the game, letting you duke it out in the world of Minecraft, others still are about a certain challenge like the Skyblock challenge or the hunger games mode, which is pretty much a battle royale all on its own. As many as there are game modes, there are more still people willing to play with you, to test themselves against you in competitions or to work together with you to build something truly special together.
Minecraft to me is the real definition of a Massively multiplayer game, since while it can be quite fun playing on your own, without the influence of others, there's a whole other world when you start playing with other people. The amount of interactions you can have, you can build friendships, you can compete with your rivals, in some game modes you can even get married. This is why, in my opinion, Minecraft is one of the best games I have ever played.