Earlier quoted context omitted.
Because it wasn't real. Cypher says ignorance is bliss but that's the opposite that everyone else feels. They'd rather eat slop they know is real. Even the suggestion of trying to go back to the Matrix willingly was seen as a betrayal. Mouse offers to set up a private meeting between Neo and the Lady in Red but he doesn't accept and we never heard of anyone else accepting.
But indeed, if they had the _option_ to escape from reality from time to time, people like Cypher would be happier (I know I am when I do it :D). Indeed the irony of partaking precisely in the thing that you fight against is there; however, what they actually fight against is the enslavement by it, not the casual enjoyment of it: it's bad to be a medieval serf forced to toil the soil, it's quite another thing to play…
Cypher: You know, I know this steak doesn’t exist. I know that when I put it in my mouth, the Matrix is telling my brain that it is juicy and delicious. After nine years, you know what I realize? Ignorance is bliss.
Agent Smith: Then we have a deal?
Cypher: I don’t want to remember nothing. Nothing. You understand? And I want to be rich. You know, someone important. Like an actor.
Agent Smith: Whatever you want, Mr. Reagan.
Cypher: Okay. I get my body back into a power plant, re-insert me into the Matrix, I’ll get you what you want.
Cypher didn't just want to eat fake steak, to him that wasn't enough. Ignorance is bliss. He wanted to eat fake steak and not know that it was fake.