"Religion is opium for the people", should be read as: "anything that gives respite, comfort, any form of escapism, any form of hope, any purpose and meaning is evil because it sedates the revolutionary will of the people".

Entertainment is inherently good - it gives hope, it's uplifting, it's escapism. And therefore it is evil.

All that is good, is evil, because The Truth is that the world is in a state of perpetual and untolerable and unjust exploitation and oppression, and the evidence for that is inequity. This is the core seed of marxism. It's the zeroeth law.

The first law is that all men are perfectly equal. Blank slate theory.

The second law is that consequentialism is the one true ethical framework.

The third law is that everything can, at it's essence, be reduced to a power struggle.

Take away everything from people that makes them happy, so they are forced to confront the harshness of reality, and to paraphrase; through their misery develop the hatred and sadism neccessary to rise up and destroy literally everything.

So that utopia can be rebuilt from the ashes of the old. Where people are equal in every way, and that equality is upheld through a constant awareness about the forces of inequity. I.e. the mental load will forever increase until the ideology consumes every waking thought - and in return we will have acheved material parity.

Everything that comes after "destroy everything" is murky however. And it's not at all clear how competing interests will harmonize, even if the laws were true, which of course they are not.

It's why true believers are so miserable, hateful, sadistic, vengeful, angry, full of rage and suffering to the core. It's by design. They're fully in the grips of the ideology, and have reached the endstage of marxist ideological posession.

It's also why every prediction leftists make turns out to be wrong. Their premises and axioms are wrong.

TL;DR

It's because they want you to suffer.