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The film demonstrates that Morpheus and the other liberated humans can enter the Matrix while in disguise, and in addition can generate their own virtual environments (eg, Neo and Morpheus sparring).
Doesn't Cypher have plenty of opportunities to enjoy a virtual steak and the other amenities of virtual life without betraying Morpheus?
The transcript for "The Matrix" makes Cypher's motives pretty clear;
CYPHER : I don't want to remember nothing. Nothing! You understand? And I want to be rich. Someone important. Like an actor. You can do that, right?
AGENT SMITH : Whatever you want, Mr. Reagan.
Cypher takes a deep drink of wine.
CYPHER : All right. You get my body back in a power plant, reinsert me into the Matrix and I'll get you what you want.
and
Cypher: I know what you're thinking, 'cause right now I'm thinking the same thing. Actually, I've been thinking it ever since I got here: Why oh why didn't I take the blue pill
and
Cypher: You know, for a long time, I thought I was in love with you. I used to dream about you. You're a beautiful woman, Trinity. Too bad things had to turn out this way.
Also, you can never underestimate the appeal of a really good steak;
He wanted to reset his memory and forget about the Matrix. There is no evidence that the "liberated humans" could do that to themselves. It was also implied that he would be inserted in the Matrix as President Ronald Reagan (see, for example this).
According to Wikipedia:
Cypher has grown tired of the Man/Machine war within the real world. Harshly disillusioned with the grim existence of living on a hovercraft and tired of following Morpheus and his blind faith in the Prophecy, he has begun to wish that he had never learned the truth and left the Matrix in the first place.
I agree with what the other answers are saying about why he wants to go back to the blissful state of the "blue pill".
As for why he betrays Morpheus, what he says when he reveals his betrayal indicates that he resents and feels betrayed by Morpheus.
Cypher: "He lied to us, Trinity. He tricked us. If you'da told us the truth, we woulda told you to shove that red pill right up your ass."
Trinity: "That's not true, Cypher, he set us free."
Cypher: "Free? You call this free? All I do is what he tells me to do. If I had to choose between that and the Matrix, I choose the Matrix."
He feels like a slave to the person who has tricked him into a dystopian reality.
Because metaphor.
Neo represents every teenage boy; you are leet, you discover something secret but world-changing, you are initiated into a new world and the scales fall from your eyes. It is a metaphor for becoming an adult.
Cypher represents defeatism, the impulse to give up and run away and be turned back into a little boy. He is the embodiment of the moment after you are rejected by a potential partner for the first time. I want to go home.
Morpheus, by contrast, is the father. He is the one who affirms Neo's maturation and believes completely in him.
Why does Cypher betray Morpheus? Because Morpheus' role to support the rite of passage into adulthood is worthless if it is better to remain a child, and Cypher rejects the truth. Also, as an allegory it illustrates how futile it is to attempt to return to the child state; once we know, we cannot unknow.
Following the logic, this allegory is telling us that hedonism (satisfying the senses) is not sufficient for happiness; the story embraces purpose (and even ideas of destiny) over sensuality.
Certain physical pleasures like steak could presumably be reproduced in private simulations, but I think the restaurant scene was meant to illustrate his overall desire for "the good life" of a wealthy person in the Matrix, and a lot of the supposed appeal of great wealth is interpersonal in nature, not just physical--like the desire for a shortcut to getting the respect of lots of people (including glamorous wealth-seeking mates), getting into certain high-profile "in-crowds", paying people for creative services like DJing your party or painting your portrait, being able to buy influence in politics or the media, etc. None of these would likely be very satisfying if he was just in a private simulation with a bunch of simple simulated people like the woman in the red dress (and even if Zion had the ability to create more genuinely intelligent simulated people, they probably wouldn't because of fear that they would take the side of the machines in the Matrix and rebel from within).
Edited to add that this section of the Cypher's lines that Richard quoted above supports the idea that this was at least part of Cypher's motivation: "And I want to be rich. Someone important. Like an actor. You can do that, right?"
CYPHER : I don't want to remember nothing. Nothing! You understand? And I want to be rich. Someone important. Like an actor. You can do that, right?
AGENT SMITH : Whatever you want, Mr. Reagan.
Cypher takes a deep drink of wine.
CYPHER : All right. You get my body back in a power plant, reinsert me into the Matrix and I'll get you what you want.
Surely his reasoning is flawed when he says he wants to remember nothing, is getting your memory totally wiped not the same as being killed? The Cypher who wakes up after such a procedure shares nothing with the old Cypher except his body
