Why should I play nethack in 2020 when I could play Dwarf fortress or Caves of Qud instead :/
Because all three of those games play very different from one another. I don't know what your tastes are, but NetHack most certainly still has its place among the modern RL scene.
If you are referring to the Adventurer mode of DF, at this point it is more about trying to create your own stories and test the game engine; it is another thing entirely.
Caves of Qud plays more like ADOM in that it has a main story line that never changes and it has an open world to roam.
NetHack is different from both of these in that it focuses on a single dungeon and complex interactivity between all of the moving parts within. It's often casually referred to as a "puzzle game", as there are a lot of unique enemies and encounters that usually have to be dealt with in a certain way.
Anyways, the short answer to your question is this: NetHack offers a type of gameplay that is not present in CoQ or DF.