Because if the DCEU demands you to watch other Batman stuff first in order to follow it better, it is NOT strong and self-contained enough to stand on it's own feet.

The DCEU doesn't demand this though. There is nothing else to watch to understand this Batman better because there are no prequels. You're condemning the DCEU for hypothetically having additional viewing material while insisting that it needs additional viewing material.

I can understand that some people will want to see his adventures prior to BvS, but really everything you need to know to follow the story is presented in that movie. His parents were murdered and he started dressing like a Bat to fight crime, his partner was presumably killed by The Joker and that sent him down a dark and violent path that culminated in him being manipulated by Lex into fighting Superman. None of that comes from the Burton movies, or the Nolan movies, or BtAS, it's all conveyed in BvS. Is there more backstory to flesh out? Sure. But to say that this interpretation of Batman is essentially incomprehensible to non-comic readers seems kind of hyperbolic.